Nine patients (median age, 58; range: 37-74) with relapsed de novo acute myeloid leukemia (AML) (3), AML after prior myelodysplastic syndrome (MDS) (4), or MDS (2) were treated with 2-20 x 10(6) U/m2/day (1-10 mg/m2/day) of recombinant human interferon gamma (rIFN gamma; Biogen) on a 14-day continuous intravenous infusion schedule. The two patients who received the initial dose of 20 x 10(6) U/m2/day (1.0 mg/m2/day) could only tolerate 6 days of therapy because of severe hepatotoxicity. Two patients who received the revised starting dose of 10 x 10(6) U/m2/day also could not complete a full course of rIFN gamma due to renal failure in one case and pulmonary deterioration in the other. A reversible dose-related rise in SGOT was seen in six patients. All patients developed a severe flu-like syndrome characterized by myalgias and fevers. These toxicities were not associated with detectable serum levels of tumor necrosis factor (TNF). Although blasts from three of five assessable patients displayed increased expression of the Ia (HLA-DR) antigen, there were no hematological responses. Steady-state rIFN gamma plasma levels in patients who tolerated a complete infusion were < 40 U/ml, a concentration below that required to induce differentiation of myeloid leukemic cell lines in vitro. We conclude that continuous infusions of rIFN gamma at doses as low as 2 x 10(6) U/m2/day are poorly tolerated in patients with AML and MDS; the maximum tolerated dose is approximately 2 x 10(6) U/m2/day.