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.