Statistics from the Connecticut Tumor Registry from 1979 to 1988 were examined, and individual medical records from 1979 to 1983 were also reviewed. Three hundred nineteen medical records were available for review, documenting 220 cases of ductal carcinoma in situ and 102 cases of lobular carcinoma in situ. In 1979, there were 33 new cases of ductal carcinoma in situ reported to the Connecticut Tumor Registry, representing 1.8% of all breast cancers. There has been a yearly increase in ductal carcinoma in situ, with 200 new cases, or 7.4% of all breast cancers, reported in 1988. Forty-eight (22%) of 217 patients with ductal carcinoma in situ had bilateral breast involvement with ductal carcinoma in situ or an invasive breast cancer. Ten (83%) of 12 mastectomy specimens from patients with ductal carcinoma in situ who presented with nipple discharge demonstrated residual tumor, suggesting a more diffuse involvement. Two of the three reported recurrences involved nipple discharge. Thirty-seven (16.8%) of the 220 patients with ductal carcinoma in situ and six (5.9%) of the 102 patients with lobular carcinoma in situ were diagnosed as having another unrelated cancer. Ongoing clinical trials will direct optimum therapy for patients increasingly diagnosed as having ductal carcinoma in situ.