This report summarizes the experience of the Joint Center for Radiation Therapy (JCRT) in treating patients with clinical stage I and II breast cancer with conservative surgery and radiation therapy. The study population consisted of 1396 patients treated between 1968 and 1985. All patients underwent a gross excision of the tumor and received breast irradiation (with or without nodal irradiation) including a "boost" to bring the primary tumor site to a total dose of at least 60 Gy. The method of treatment evolved over the study-time period. During the interval from 1968 to 1982, patients typically underwent a limited gross excision of the tumor without regard to the microscopic margins of resection. During the period 1983 to 1985, film-screen mammography, inking of specimen margins, and reexcisions for inevaluable or involved margins were more commonly performed. With a median follow-up of 80 months, the 5-year crude rate of failure in the breast (as the first site of failure) was 8% (106/1396) and the crude rate of regional nodal/distant failure (as the first site of failure) was 16% (228/1396). The time-course of failures in the breast was protracted, occurring at a fairly constant rate over the first 7 years after treatment, but still seen beyond that point. Most recurrences in the breast (75%) developed at or near the original tumor site. The most important risk factor for developing a breast recurrence was the presence of an extensive intraductal component in the tumor. The cosmetic results following treatment were excellent or good in the majority of patients (87%) and were most adversely affected by extensive surgery.(ABSTRACT TRUNCATED AT 250 WORDS)