Achieving a healthy weight is recommended for all breast cancer survivors. Previous research on postdiagnosis weight change and mortality had conflicting results. We examined whether change in body weight in the 18 months following diagnosis is associated with overall and breast cancer-specific mortality in a cohort of n = 12,590 stage I-III breast cancer patients at Kaiser Permanente using multivariable-adjusted Cox regression models. Follow-up was from the date of the postdiagnosis weight at 18 months until death or June 2015 [median follow-up (range): 3 (0-9) years]. We divided follow-up into earlier (18-54 months) and later (>54 months) postdiagnosis periods. Mean (SD) age-at-diagnosis was 59 (11) years. A total of 980 women died, 503 from breast cancer. Most women maintained weight within 5% of diagnosis body weight; weight loss and gain were equally common at 19% each. Compared with weight maintenance, large losses (≥10%) were associated with worse survival, with HRs and 95% confidence intervals (CI) for all-cause death of 2.63 (2.12-3.26) earlier and 1.60 (1.14-2.25) later in follow-up. Modest losses (>5%-<10%) were associated with worse survival earlier [1.39 (1.11-1.74)] but not later in follow-up [0.77 (0.54-1.11)]. Weight gain was not related to survival. Results were similar for breast cancer-specific death. Large postdiagnosis weight loss is associated with worse survival in both earlier and later postdiagnosis periods, independent of treatment and prognostic factors. Weight loss and gain are equally common after breast cancer, and weight loss is a consistent marker of mortality risk. Cancer Epidemiol Biomarkers Prev; 26(1); 44-50. ©2016 AACR SEE ALL THE ARTICLES IN THIS CEBP FOCUS SECTION, "THE OBESITY PARADOX IN CANCER EVIDENCE AND NEW DIRECTIONS".