In lymph-node-negative invasive breast cancer patients<55 years, the proliferation marker mitotic activity index (MAI) has previously been shown to be the strongest prognosticator. In studies without age definition, MAI was not strongly prognostic. We investigated the age dependency of the prognostic value of proliferation for distant metastasis-free (MFS) and overall cancer-related survival (OS) in 1004 histologically diagnosed T1-3N0M0 invasive breast cancers (n=516, <55 years; n=322, 55-70 years; n=166, >70 years) without systemic adjuvant therapy and long follow-up (median: 108 months). The MAI decreases with age and the prognostic value of MAI varied by age group. For patients<55 years, hazard ratios (HR) for MAI>or=10 versus<10 for MFS and OS were 3.1 and 4.4, respectively (P<.0001 for both), but only 1.9 and 1.9 (P=.004 and .006) for patients aged 55-70 years, while over 70 years, MAI was not significant (P=.11). The prognostic value of proliferation was age-dependent. Prognostic breast cancer studies must clearly indicate the age group being studied.