Item 12(b).
| CERTIFICATION UNDER SECTION 906 OF SARBANES-OXLEY ACT OF 2002 | ||
| Name of Issuer: T. Rowe Price Mid-Cap Growth Fund | ||
| In connection with the Report on Form N-CSR for the above named Issuer, the undersigned hereby | ||
| certifies, to the best of his knowledge, that: | ||
| 1. | The Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities | |
| Exchange Act of 1934; | ||
| 2. | The information contained in the Report fairly presents, in all material respects, the financial | |
| condition and results of operations of the Issuer. | ||
| Date: August 17, 2016 | /s/ Edward C. Bernard | |
| Edward C. Bernard | ||
| Principal Executive Officer | ||
| Date: August 17, 2016 | /s/ Catherine D. Mathews | |
| Catherine D. Mathews | ||
| Principal Financial Officer | ||