The time course and prognosis of local recurrence was studied for 1593 stage I-II patients treated by macroscopically complete excision of the primary tumor with or without axillary dissection, followed by megavoltage radiotherapy (mean tumor bed dose 78 Gy). The actuarial cumulative breast failure rate was 7% at 5, 14% at 10, 18% at 15 and 20% at 20 years. The proportion of recurrences occurring at a distance from the original tumor bed increased with time, most recurrences after 10 years being "new tumors". Of 178 breast recurrences observed, 159 (89%) were operable. Overall survival following operable relapse was 69% at 5 and 57% at 10 years. Prognosis after local failure depended both upon the disease-free interval and the extent of the recurrence. Late recurrences diagnosed after 5 years had an excellent outlook (5-year survival 84%). Small recurrences confined to the breast had a favorable prognosis, even if occurring before 5 years. Local control after salvage mastectomy (88% at 5 years) was better than after conservative salvage surgery. This extensive experience illustrates that recurrence in the breast after conservation therapy is characterized by a protracted course, and is not associated with the poor prognosis attributed to local relapse after mastectomy. The diagnosis and surgical treatment of mammary recurrence is an important component of the breast-conserving strategy.