One hundred and twenty-one cases of ductal carcinoma in situ, including 26 cases with T1a invasive carcinoma, were reviewed. Seventy-nine patients (65%) were treated by mastectomy and 42 (35%) had conservative surgery. Ductal carcinoma in situ was classified as well differentiated (11%), intermediately differentiated (22%) or poorly differentiated (67%) according to nuclear morphology and the presence or absence of cell polarization. Poorly differentiated lesions were significantly larger than intermediately and well differentiated lesions (P = 0.03 and P = 0.01, respectively) and were significantly associated with the presence of extensive necrosis, marked periductal inflammation and periductal fibrosis (P < 0.0001). Invasive carcinoma was more common in the poorly differentiated group (25% compared with 18% in the intermediate group and 8% in the well differentiated group) but this was not statistically significant. The spectrum of differentiation was similar in symptomatic and mammographically detected ductal carcinoma in situ. Clinical follow-up was available in 90 patients (median period 45 months in patients who had undergone mastectomy and 23 months in those who had conservative surgery). Two incidences of recurrent local disease were recorded in the mastectomy group: one patient had well differentiated and the other poorly differentiated ductal carcinoma in situ. No local recurrences were observed in the conservative surgery group, possibly reflecting the shorter follow up period. All histological grades of ductal carcinoma in situ have the potential to progress to invasive carcinoma and mastectomy does not guarantee a cure.