Twenty-four patients with acute promyelocytic leukemia were reviewed. Group I, treated between July 1970 and September 1973, received arabinosylcytosine and 6-thioguanine, and there was one complete remission, with 4/7 dying during induction with intracerebral hemorrhages, and 2/7 dying within one month. Group II, treated between May 1974 and March 1975, received daunomycin and arabinosylcytosine without heparin and 2/8 went into remission, with 6/8 dying during induction, 5 with intracerebral hemorrhages. Group III, treated between March 1975 and November 1976, received the identical chemotherapy as group II but with the addition of prophylactic heparin and there were 7/9 complete remissions, with 2/9 dying with intracerebral hemorrhages. In Group III there was an increased incidence of remission induction when compared to Group II (p less than .05) or when compared to Groups I and II combined (p less than .05). There was also a decreased incidence of fatal hemorrhage in Group III. This suggests that prophylactic heparin is useful during remission induction in acute promyelocytic leukemia.