We treated 109 patients with adult acute lymphoblastic leukemia (ALL) diagnosed by histochemical and immunologic techniques. Patients were excluded only for age greater than 50 years and Burkitt's leukemia. Treatment included a four-drug remission induction phase followed by alternating cycles of noncrossresistant chemotherapy and prolonged oral maintenance therapy. Eighty-eight percent of patients entered complete remission. With a median follow-up of 77 months (range, 48 to 111 months), 42% +/- 6% (SEM) of patients achieving remission are projected to remain disease-free at 5 years, and disease-free survival for all patients entered on study is 35% +/- 5%. Failure to achieve remission within the first 4 weeks of therapy and the presence of the Philadelphia chromosome are associated with a 100% risk of relapse. Remission patients with neither of these adverse features have a 48% +/- 6% probability of remaining in continuous remission for 5 years. Patients with T-cell phenotype have a favorable prognosis with 59% +/- 13% of patients achieving remission remaining disease-free compared with 31% +/- 7% of CALLA-positive patients. Intensive chemotherapy may produce prolonged disease-free survival in a sizable fraction of adults with ALL. Improved therapy is needed, especially for patients with adverse prognostic features.