The risk of developing breast cancer was investigated in 339 patients in whom histologically confirmed intraductal papilloma had been surgically removed. Follow-up ranged from 2 to 14 years, (average, 6.62 years). Ten breast cancers were observed, whereas 3 were expected on the basis of age-specific incidence rates drawn from local cancer registries (relative risk = 3.33, 95% confidence interval = 1.60-6.13). No significant difference in the relative risk was observed as far as papilloma type (single or multiple) was concerned. All breast cancers observed occurred in the same breast as the papilloma. Women in whom a benign intraductal papilloma has been excised should be carefully followed since they at are higher risk for ipsilateral breast cancer.