131 patients with Philadelphia (Ph') chromosome positive chronic myelogenous leukemia (CML) were treated with interferon (IFN) alpha or a combination of IFN alpha and IFN gamma. In study, 1, 13 not pretreated and 41 pretreated patients, 48 in chronic phase disease, 6 in accelerated phase, received 4 x 10(6) U/m2 IFN alfa-2b initially. After achievement of hematologic remission, the individually minimum effective dose was used for maintenance. There was no response in acute phase disease. Of the 48 patients with chronic phase disease, 22 achieved a hematologic remission (HR), 13 a partial HR (PHR), and 13 did not respond (NR) to IFN. No major cytogenetic response occurred in these patients, but two patients achieved a major molecular response in Southern blots, whereas PCR tests were positive. In a further randomized study, CML patients without prior therapy were treated initially with 4 x 10(6) U/m2 IFN alpha alone (arm A) or in combination with 50 micrograms IFN gamma (arm B). For maintenance, the maximum tolerable dose of IFN alpha was given (up to 10 x 10(6) U/day). Thirteen patients in arm A (54%) and 14 (56%) patients in arm B achieved a HR, 7 patients (29%) in arm A and 6 patients (24%) in arm B a PHR. No response could be induced in 4 patients (17%) of arm A and 5 patients (21%) of arm B. Major cytogenetic responses were observed in 5 (20%) patients of arm A and 5 patients (20%) of arm B. Major molecular responses were observed in 3 patients of arm A and 5 patients of arm B.(ABSTRACT TRUNCATED AT 250 WORDS)