BACKGROUND The National Surgical Adjuvant Breast and Bowel Project (NSABP) Protocol B-06, a clinical trial sponsored by the National Cancer Institute (NCI), has provided evidence of the value of lumpectomy and breast irradiation for treating women with breast cancer in an early stage. Publicity generated by the discovery that the study included fraudulent data on patients enrolled by St. Luc Hospital in Montreal aroused concern about the overall accuracy of the data and conclusions. To address this concern, the NCI conducted an audit of other participating institutions. METHODS In 1994, data on 1554 of the 1809 randomized patients (85.9 percent) enrolled by centers other than St. Luc Hospital were audited at 37 clinical sites in North America. The audit included data on eligibility, survival, disease-free survival, the length of time to a recurrence of cancer in the ipsilateral breast, and documentation of signed informed consent. RESULTS End points were assessed for all 1554 patients, and eligibility was assessed for 1507 patients; 47 patients were excluded because their forms were not complete or not returned. A total of 1429 patients had their eligibility status verified. Of a total of 7770 data points examined with respect to the number of positive nodes at base line, treatment characteristics, first events (excluding death), recurrence of cancer in the ipsilateral breast, and survival, 7577 (97.5 percent) were verified, 123 (1.6 percent) could not be verified, and 70 (0.9 percent) were discrepant with the NSABP file. Of the 1554 patients, 1340 (86.2 percent) had all audited items (including eligibility) verified, 69 (4.4 percent) had at least one discrepant item, and 113 (7.3 percent) had at least one unverified item (as a result of missing or incomplete data); 32 (2.1 percent) were not assessed for eligibility but had no other discrepant or unverifiable items. Written informed consent was documented for 1098 patients before surgery and 210 after surgery; no date appeared on the signed form for 137. The informed-consent status was not verified for 71 patients and could not be determined for 38. The rates of verification of end-point data and documentation of written informed consent were similar among the total-mastectomy group, the lumpectomy group, and the group treated by lumpectomy and breast irradiation. CONCLUSIONS The audit confirms the adequacy of the data on which the reanalysis of Protocol B-06 and the results after 12 years of follow-up are based.