The complete hematological remission (CHR) rate, duration of remission and survival were studied in relation to age, peripheral blast cell (PBC) count, presence or absence of tumor masses, cytological type, and treatment in 650 patients with acute lymphoblastic leukemia. Prognostic factors were considered separately and divided into prognostic classes. Age and PCB count correlated with both the rate and the duration of CHR. This correlation was still observed for more recent treatment schedules though it appears to be becoming progressively less significant. Meningeal relapses were more common in patients less than 1 year old and in those with a high PCB count. It is suggested that stratification of patients according to such factors as age, PCB count, presence or absence of tumor, and cytological type might be necessary for the design of new treatment protocols and for the evaluation of their results.