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Data from a 30-year follow-up study of 606 women given radiotherapy for acute postpartum mastitis and two non-irradiated control groups were reanalyzed to determine whether bias could account for the reported increased risk of benign and malignant breast tumors in the irradiated population. The biases examined were comparison group selection (i.e. selection bias); differential medical verification of breast tumors between exposed and unexposed groups; differential detection of breast tumors; and confounding. Overall age-adjusted relative risk (RR) estimates for breast cancer (RR = 2.0; 95% CL:1.3-3.1) and for benign neoplastic (RR = 3.5; 95% CL:1.8-6.8) and non-neoplastic breast tumors (RR = 1.5; 95% CL: 1.2-2.1) were compared to those obtained after adjustment for the above biases. The increased risk for breast cancer among the irradiated women could not be accounted for by any of the biases examined, supporting previous reports based on this cohort. However, the suggested association of radiation with benign breast tumors may represent a selection bias since the RR estimate for non-neoplastic breast tumors was no longer statistically significant (RR = 1.2; 95% CL:0.8-1.6) while that for neoplastic breast lesions declined to 2.0 (95% CL:1.1-1-4.0) when non-irradiated women with acute postpartum mastitis were used as the comparison group instead of the non-irradiated female siblings of the exposed. This finding suggests that acute postpartum mastitis may predispose a woman to later developing a benign breast tumor irrespective of whether or not radiotherapy was used to treat this condition.(ABSTRACT TRUNCATED AT 250 WORDS)