The authors used data from a population-based case-control study of breast cancer in women aged < or = 44 years (cases, n = 975; controls, n = 866) conducted between 1994 and 1996 in three counties of western Washington state to assess the validity and reliability of reported perinatal factors. For a sample of participants, exposure information from self-administered questionnaires was validated with information from birth certificates (cases, n = 378; controls, n = 283). Detailed information regarding perinatal characteristics of their daughters was also collected from subjects' mothers (case mothers, n = 510; control mothers, n = 436) to assess the reliability of subjects' reporting of these events. Although reporting of birth weight by subjects (cases, r = 0.83; controls, r = 0.80) and their mothers (case mothers, r = 0.89; control mothers, r = 0.84) was highly correlated with the birth certificates, there was differential measurement error by subjects; cases reported birth weight accurately on average, but controls tended to underestimate their birth weight. Agreement between the subject and mother report was excellent for birth weight (cases, r = 0.85; controls, r = 0.87) and good for other perinatal factors, but birth order and maternal diethylstilbestrol use were underreported among cases and reported accurately among controls. Differential measurement error of birth weight by case-control status resulted in biased odds ratios for breast cancer risk.