Although major progress has been made in the treatment of childhood leukemia, the optimal chemotherapy of acute lymphoblastic leukemia (ALL) in adults has been unclear. In addition, the value of central nervous system prophylaxis (CNS-P) in adults has been assumed, but not established in a systematic fashion. The Southeastern Cancer Study Group has completed a prospective study in which the use of vincristine plus low-dose methotrexate and high-dose prednisone in adult acute lymphoblastic leukemia has produced an 80% (79/99) complete remission rate in patients age 15 yr and over. Younger patients had a significantly higher remission rate but no increase in remission duration. This induction regimen was associated with minimal toxicity. Random assignment to CNS-P or to no prophylaxis, after a multidrug consolidation regimen, has demonstrated a significant prolongation of CNS relapse-free interval (p=0.008) in favor of CNS-P. CNS-P did not improve hematologic remission duration or survival. All complete remitters were maintained on mercaptopurine, methotrexate, and cyclophosphamide with pulses of prednisone and vincristine; the median time from remission to either hematologic or CNS relapse was 19.3 mo after CNS-P, and survival for these patients was 26.1 mo. We conclude that our current induction regimen is highly effective in adult ALL and that CNS-P prophylaxis is indicated in such patients.