In a mammography screening programme for the early detection of breast cancer 66 cases of ductal cancer in situ of the breast (DCIS) were diagnosed between 1978 and 1984 and prospectively followed up. In the beginning of the study period, DCIS was treated by mastectomy without axillary clearance but sector resection has been performed increasingly. Since 1982 the latter procedure has become standard treatment. Radical resection was confirmed by specimen X-ray and histopathological examination of whole organ sections. No postoperative radiotherapy was given. Twenty-eight women had mastectomy and 38 had sector resection. The median follow-up times were 77 and 60 months respectively. In the mastectomy group postoperative monitoring did not reveal any local recurrence but one contralateral new invasive cancer was discovered. In the sector resection group five recurrences were found. Three of the latter were new cases of DCIS and two appeared as small invasive cancers (stage I). Mastectomy will cure ductal cancer in situ but has a greater morbidity. Sector resection has been established as the method of choice in stage I invasive cancer and is probably also safe in DCIS. The possible beneficial effect of postoperative local radiotherapy after sector resection for DCIS is currently being analysed in a randomized trial which started in Sweden in 1987.