UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 

SCHEDULE 13G 

Under the Securities Exchange Act of 1934 

Amendment No.: 7* 

Name of Issuer: Masimo Corporation 

Title of Class of Securities: Common Stock 

CUSIP Number: 574795100 

Date of Event Which Requires Filing of this Statement: 12/31/2017 

Check the appropriate box to designate the rule pursuant to which this 
Schedule is filed. 

[X] Rule 13d-1(b) [ ] Rule 13d-1(c) [ ] Rule 13d-1(d) 

* The remainder of this cover page shall be filled out for a reporting 
person's initial filing on this form with respect to the subject class 
of securities, and for any subsequent amendment containing information 
which would alter the disclosures provided in a prior cover page. 

The information required in the remainder of this cover page shall not 
be deemed to be "filed" for the purpose of Section 18 of the Securities 
Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of 
that section of the Act but shall be subject to all other provisions of 
the Act (however, see the Notes). 

CUSIP No.: 574795100 

1. NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE 
PERSON Janus Capital Management LLC EIN #75-3019302 

2. CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP a. ___ b. ___ 3. SEC 
USE ONLY 

4. CITIZENSHIP OR PLACE OF ORGANIZATION Delaware 

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH: 

5. SOLE VOTING POWER 3,069** 

6. SHARED VOTING POWER 0** 

7. SOLE DISPOSITIVE POWER 3,069** 

8. SHARED DISPOSITIVE POWER 0** 

9. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 3,069** 

10. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES 
Not applicable 

11. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 0.0%** 

12. TYPE OF REPORTING PERSON IA, HC 

** See Item 4 of this filing 

Item 1. 

(a). Name of Issuer: Masimo Corporation ("Masimo") 

(b). Address of Issuer's Principal Executive Offices: 

52 Discovery Irvine, CA 92618 

Item 2. 

(a).-(c). Name, Principal Business Address, and Citizenship of 
Persons Filing: 

(1) Janus Capital Management LLC ("Janus Capital") 151 Detroit Street 
Denver, Colorado 80206 Citizenship: Delaware 

(d). Title of Class of Securities: Common Stock 

(e). CUSIP Number: 574795100 

Item 3. 

This statement is filed pursuant to Rule 13d-1 (b) or 13d-2(b) and the 
person filing, Janus Capital, is an investment adviser in accordance 
with Section 240.13d-1(b)(ii)(E) as well as a parent holding 
company/control person in accordance with Section 240.13d-1(b)(ii)(G). 
See Item 4 for additional information. 

Item 4. Ownership 

The information in items 1 and 5 through 11 on the cover page(s) on 
Schedule 13G is hereby incorporated by reference. 

Janus Capital has a direct 97.11% ownership stake in INTECH Investment 
Management ("INTECH") and a direct 100% ownership stake in Perkins 
Investment Management LLC ("Perkins"). Due to the above ownership 
structure, holdings for Janus Capital, Perkins and INTECH are aggregated 
for purposes of this filing. Janus Capital, Perkins and INTECH are 
registered investment advisers, each furnishing investment advice to 
various investment companies registered under Section 8 of the 
Investment Company Act of 1940 and to individual and institutional 
clients (collectively referred to herein as "Managed Portfolios"). 

As a result of its role as investment adviser or sub-adviser to the 
Managed Portfolios, Janus Capital may be deemed to be the beneficial 
owner of 3,069 shares or 0.0% of the shares outstanding of Masimo Common 
Stock held by such Managed Portfolios. However, Janus Capital does not 
have the right to receive any dividends from, or the proceeds from the 
sale of, the securities held in the Managed Portfolios and disclaims any 
ownership associated with such rights. 

Item 5. Ownership of Five Percent or Less of a Class 

The Managed Portfolios, set forth in Item 4 above, have the right to 
receive all dividends from, and the proceeds from the sale of, the 
securities held in their respective accounts. 

This statement is being filed to report the fact that the reporting 
persons have ceased to be the beneficial owners of more than five 
percent of the class of securities. 

These shares were acquired in the ordinary course of business, and not 
with the purpose of changing or influencing control of the Issuer. 

Item 6. Ownership of More than Five Percent on Behalf of Another Person 

Not applicable. 

Item 7. Identification and Classification of the Subsidiary Which 
Acquired the Security Being Reported on by the Parent Holding Company 

Not applicable. 

Item 8. Identification and Classification of Members of the Group 

Not applicable. 

Item 9. Notice of Dissolution of Group 

Not applicable. 

Item 10. Certification 

By signing below I certify that, to the best of my knowledge and belief, 
the securities referred to above were acquired in the ordinary course of 
business and were not acquired for the purpose of and do not have the 
effect of changing or influencing the control of the issuer of such 
securities and were not acquired in connection with or as a participant 
in any transaction having such purposes or effect. 

SIGNATURES 

After reasonable inquiry and to the best of my knowledge and belief, I 
certify that the information set forth in this statement is true, 
complete and correct. 

JANUS CAPITAL MANAGEMENT LLC 

By /s/ Brennan Hughes 2/6/2018 Brennan Hughes, 
Date Senior Vice President and Treasurer 




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