To determine the frequency, pattern, and time course of tumor recurrence in the breast, the outcome of 38 women with ductal carcinoma in situ (DCIS) treated with conservative surgery and radiation therapy between 1976 and 1985 was studied. Surgery typically consisted of local excision without evaluation of the microscopic margins of resection. The median radiation dose to the tumor site was 6400 cGy. With a median follow-up time of 81 months, eight patients (21%) have experienced a recurrence in the breast. The time course to recurrence was protracted in some cases, with failures occurring at 17, 27, 43, 63, 71, 83, 92, and 104 months. The 5-year and 8-year actuarial rates of tumor recurrence in the breast were 8% and 27%, respectively. Seven patients had a recurrence at or near the primary tumor site, four with invasive carcinoma, and one had an invasive recurrence at a site elsewhere in the breast. No clinical or pathologic factor was significantly associated with an increased risk of recurrence, but the number of patients in the study population was small. The authors reached the following conclusions for patients with DCIS treated with conservative surgery and radiation therapy without careful mammographic and pathologic evaluation: (1) recurrence in the breast may be seen in at least one fifth of the patients; (2) recurrence typically occurs at or near the primary site; and (3) recurrence can occur long after treatment. Careful mammographic and pathologic assessment may be useful in reducing the local recurrence rate and should be considered essential if patients are considered for conservative surgery and radiation therapy.