Inverse associations have been reported between birthweight and subsequent mortality from circulatory disease and diabetes among women. In the current study, we assessed whether perinatal factors were associated with mortality from breast cancer. This follow-up study consists of breast cancer cases who participated in two population-based case-control studies of breast cancer in women under age 45 years conducted between 1983 and 1992 in three western Washington counties. This analysis is restricted to the 1,024 cases or their proxies who completed a supplementary questionnaire on perinatal factors from 1994 to 1996. The mean and median length of follow-up among living cohort members were 153 and 148 months, respectively. Relative to women who were firstborn, women who were born second or higher in the birth order seemed to have lower mortality from breast cancer [hazard ratio (HR), 0.2; 95% confidence interval (95% CI), 0.2-0.3]. In contrast, maternal age of > or =35 years (HR, 1.7; 95% CI, 1.1-2.8) was associated with higher breast cancer mortality relative to a maternal age of <25 years. Birth order modified the effect of maternal age on mortality from breast cancer (P = 0.03). There was evidence of increased breast cancer mortality for birthweight of > or =4,000 g (HR, 1.8; 95% CI, 1.0-3.1) and twin membership (HR, 2.5; 95% CI, 1.0-6.2). The protective effect of being born second or higher in the birth order against breast cancer mortality regardless of maternal age is striking and needs to be confirmed in future studies.