Findings from the National Surgical Adjuvant Breast and Bowel Project (NSABP) trials evaluating three different regimens of adjuvant chemotherapy (L-PAM, L-PAM + 5-FU, L-PAM + 5-FU + MTX) in patients with primary breast cancer and positive axillary nodes indicate that each regimen has significantly contributed toward achieving the initial goal of such therapy, namely to diminish or prevent treatment failure in all or major subsets of patients during the first two years following operation when women are at greatest risk for a recurrence. Because of this hazard, chemotherapy was administered in all protocols for two years. Findings were examined at the end of the first year of therapy and at the termination of the second year for those who entered that year of therapy disease-free in order to determine whether the second year of treatment contributed a benefit beyond that achieved from the first year of therapy. A reduction in the incidence of treatment failure was evident in every subgroup of patients at completion of the first year of therapy. There was evidence of added improvement during the second year of treatment in patients aged 49 years or younger but not in those aged 50 years or older. Despite the finding, it is not possible from these studies to be absolutely certain that a second year of therapy is or is not advantageous. Findings obtained to date from the three studies indicate that patients completing two years of chemotherapy who are disease-free display a subsequent treatment failure rate that is no greater than that observed in untreated patients who survived two years without recurrence. Consequently, any advantage in disease-free survival observed at completion of therapy has been subsequently sustained for several years, suggesting this represents a real decrease rather than a postponement of treatment failure.