Background Senescent cells, which express p16INK4a, accumulate with aging and contribute to age-related pathology. To understand whether cytotoxic agents promote molecular aging, we measured expression of p16 INK4a and other senescence markers in breast cancer patients treated with adjuvant chemotherapy. Methods Blood and clinical information were prospectively obtained from 33 women with stage I to III breast cancer at four time points: before anthracycline-based chemotherapy, immediately after anthracycline-based chemotherapy, 3 months after anthracycline-based chemotherapy, and 12 months after anthracycline-based chemotherapy. Expression of senescence markers p16INK4a and ARF mRNA was determined using TaqMan quantitative reverse-transcription polymerase chain reaction in CD3 + T lymphocytes, telomere length was determined by Southern analysis, and senescence-associated cytokines were determined by enzyme-linked immunosorbent assay. Findings were independently assessed in a cross-sectional cohort of 176 breast cancer survivors enrolled a median of 3.4 years after treatment; 39% previously received chemotherapy. All statistical tests were two-sided. Results In prospectively analyzed patients, expression of p16 INK4a and ARF increased immediately after chemotherapy and remained elevated 12 months after treatment. Median increase in log2p16 INK4a was 0.81 (interquartile range = 0.28-1.62; Wilcoxon signed-rank P <. 001), or a 75% absolute increase in expression, equivalent to the increase observed over 14.7 years of chronological aging. ARF expression was comparably increased (P <. 001). Increased expression of p16INK4a and ARF was associated with dose-dense therapy and hematological toxicity. Expression of two senescence-associated cytokines (VEGFA and MCP1) was durably increased by adjuvant chemotherapy. Telomere length was not affected by chemotherapy. In a cross-sectional cohort, prior chemotherapy exposure was independently associated with a log2-increase in p16INK4a expression of 0.57 (repeated measures model, P <. 001), comparable with 10.4 years of chronological aging. Conclusions Adjuvant chemotherapy for breast cancer is gerontogenic, inducing cellular senescence in vivo, thereby accelerating molecular aging of hematopoietic tissues. © The Author 2014. Published by Oxford University Press. All rights reserved.