During long-term interferon alpha-2b (IFN) therapy of Philadelphia chromosome-positive chronic myelogenous leukemia (CML) patients, short-term effects of tumor necrosis factor alpha (TNF) on peripheral leukocyte counts, as well as cortisol and corticotropin (ACTH) release were studied. TNF (40-160 micrograms/m2) was given as a 2-h infusion on 5 consecutive days every 3 weeks, in addition to s.c. daily IFN injections (4 mio U/m2), to four (two male/two female) patients, who had been treated for more than 8 months with IFN and additionally for 0-7 months with TNF. Leukocyte counts, cortisol, and ACTH were determined at 30-min intervals between 4 p.m. and midnight. Profiles were determined the day before and on day 1 of TNF therapy. Leukocyte numbers decreased 30 min after start of TNF administration and increased 30-60 min later with a rebound until the next TNF application. The increase of leukocyte counts was due mostly to neutrophil granulocytes. ACTH levels increased 30 min, cortisol 60 min, and leukocyte counts 90 min after start of TNF infusion. Metopirone, an inhibitor of cortisol synthesis given to one patient, suppressed the TNF-induced stimulation of cortisol secretion and subsequent increase of leukocyte counts, while ACTH blood levels were enhanced. It was concluded that leukocyte count increases after TNF/IFN administration might be related to TNF-evoked cortisol secretion.