ABSTRACT. Colony‐forming cells (CFU‐C) in peripheral blood and bone marrow and colony‐stimulating activity (CSA) in mononuclear peripheral white blood cells were studied at diagnosis in 87 patients with acute nonlymphoblastic leukemia (ANLL). Absence of CFU‐C in peripheral blood was more frequent in patients who did not enter remission than in those who did, and survival was significantly shorter in CFU‐C‐negative than in CFU‐C‐positive patients. No correlation was found between CFU‐C in the bone marrow and frequency of remission or survival time. Absence of CSA was significantly more frequent in patients who did not enter remission than in those who did. Only 4 of 28 patients who lacked CSA entered remission. Survival was significantly longer in CSA‐positive than in CSA‐negative patients. Thus, CSA synthesis in peripheral mononuclear blood cells appears to be a valuable prognostic factor in ANLL.