To analyse the incidence and treatment of ductal carcinoma in situ (DCIS) of the breast in the population. Retrospective, descriptive. The incidence of DCIS in the period 1975-1997 in 8 hospitals in southeastern North Brabant and northern Limburg, the Netherlands, was analysed, using data from the Eindhoven Cancer Registry. Changes in the treatment of DCIS in the period 1984-1997 were studied, using information from the Breast Cancer Documentation Project. Between 1975 and 1997, 448 cases of DCIS were detected in 442 women. After adjustment for age according to the European Standard Population, the incidence rate of DCIS increased from 0.3 per 100,000 in 1975 to 13.4 in 1997. Of the 87 patients with DCIS between 50 and 69 years of age in 1996 and 1997, 68% was detected by the screening programme. The proportion of patients with DCIS undergoing ablation of the breast was 53% during the period 1984-1989, and varied around 45% thereafter. The other patients underwent local excision. The proportion of patients receiving radiotherapy after local excision decreased from 78% in the period 1984-1989 to 24% in the period 1994-1995 and rose to 41% during the last few years. Substantial differences in the treatment of DCIS were observed between the 8 hospitals.