Nine patients with refractory chronic myelogenous leukemia and severe symptomatic thrombocytosis (greater than or equal to 1 X 10(6) platelets/mm3) were given partially purified human leukocyte interferon-alpha. A significant decline in platelet counts, from a mean (+/- SE) of 1.71 +/- 0.53 X 10(6)/mm3 to a mean of 0.52 +/- 0.24 X 10(6)/mm3 (p less than 0.01), resulted in all patients. Maintenance of low platelet counts was achieved in two patients for more than 143 and 300 days, respectively. Treatment with human leukocyte interferon-alpha was stopped in the remaining patients because of increases in the leukocyte count, toxicity, or both. Our preliminary observations suggest that human leukocyte interferon-alpha may significantly alleviate progressive thrombocytosis in advanced chronic myelogenous leukemia. Further studies of human leukocyte interferon-alpha and chemotherapeutic agents are indicated.