OBJECTIVE This study assesses the results of "current clinical practice" among 882 women treated in nine French Cancer Centers from 1985 to 1995 for pure ductal carcinoma in situ (DCIS) of the breast. METHODS Median age was 53 years (range 21-87); 177 (20%) patients underwent mastectomy (M), 190 (22%) conservative surgery alone (CS) and 515 (58%) conservative surgery with radiotherapy (CS + RT). RESULTS The crude 7-year local relapse (LR) rates were 2%, 31% and 13% among the M, CS and CS+RT subgroups (p<0.0001). All four LR after M were invasive as well as 31 (52%) out of 59 and 40 (61%) out of 66 in the CS and CS+RT groups. Distant metastases occurred in 1%, 3% and 1% of the three treatment groups. No LR factors were found in the M group. Among women treated with CS, the 7-year LR rates were 36%, 31% and 30% among women aged 40 or less, 41 to 60 and 61 or more (NS). For women treated by CS+RT, the LR rates in these age subgroups were 33%, 13% and 8%, respectively (p<0.0001). Patients with negative, positive or uncertain margins had 7-year LR rates of 26%, 56% and 29% respectively if treated with CS (p=0.02) and 11%, 23% and 9% if treated with CS+RT (p=0.0008). RT reduced LR rates by 65% in all histological subgroups, but more particularly in comedocarcinoma and mixed cribriform/papillary subgroups. The 7-year rate of contralateral breast cancer was 7%, identical in all subgroups. CONCLUSIONS Mastectomy remains the safest treatment for women with DCIS, with a 98% 7-year control rate. After conservative surgery, RT reduces very significantly LR rates, according to the NSABP B-17 and EORTC 10853 randomized trial results. The RT benefit is present in all clinical/histological subgroups, but its magnitude varies. Young age (<40 years) and incomplete excision are the most important LR risk factors.