Between Jan. 1, 1985 and Aug. 31, 1987, 62 in-situ ductal carcinomas with a predominantly comedo pattern were identified in 61 patients in British Columbia from excisional biopsy of a palpable or mammographically demonstrable lesion of a breast. The biopsies were intended to remove the lesion completely. Fifty-seven (92%) of the 61 patients required wide re-excision or total mastectomy, usually within a month of the initial biopsy. Occult invasive disease was demonstrated in 14 of the re-excision specimens (24.5%) and residual in-situ carcinoma was present in a further 24 (42.1%), giving an overall rate of residual disease of 66.6%. Axillary lymph nodes were sampled in 54 cases. Metastases were found in two cases (3.7%) and each was associated with occult infiltrating ductal carcinoma in the breast. This suggests that in-situ ductal carcinoma having a predominant comedo pattern may be more widespread and associated with a higher incidence of invasive ductal carcinoma than is generally believed.