Data from a population-based case-control study were analyzed to evaluate risk of breast cancer among women according to parental age at the time of subject birth. Between 1988-91, breast cancer cases (n = 1,253) were obtained from the statewide tumor registry in Wisconsin. Concurrently, population controls (n = 1,121) were randomly selected from driver's license lists (if under age 65) and Medicare beneficiary files (if 65-74 years). Information regarding parents' ages and breast cancer risk factors was obtained by telephone interview. Relative risk estimates were very slightly elevated with increasing maternal age, although no consistent trend of increasing risk was observed (P for trend = 0.38). No association between paternal age and breast cancer risk was observed (P for trend = 0.98). Older maternal or paternal age was not associated with risk among any of the subgroups examined, except for daughters who had late age at first birth. These findings are consistent with the majority of studies that have found little or no association between parental age and breast cancer risk.