Twelve male patients, eight with the acquired immunodeficiency syndrome (AIDS) and four with AIDS related complex (ARC), who had zidovudine associated neutropenia (less than 1 x 10(9) neutrophils/l) were treated with recombinant human granulocyte colony-stimulating factor (G-CSF) in a phase I/II study. Treatment consisted of daily subcutaneous injections with G-CSF in a weekly increasing dose of 0.4, 2, 5 or 10 micrograms/kg body weight until a neutrophil count of more than 3 x 10(9) neutrophils/l was observed. This effective dose was continued for up to 4 weeks, followed by 4 weeks observation period without G-CSF treatment. Two patients (both with ARC) reached target neutrophil counts at the lowest G-CSF dose, whereas nine patients needed 2 micrograms/kg. One patient discontinued treatment before he reached target neutrophil counts. Mean (+/- SD) neutrophil counts before and after 1 and 4 weeks of effective dose treatment were 0.65(+/- 0.188) x 10(9), 6.016(+/- 2.595) x 10(9) and 5.54(+/- 4.237) x 10(9)/l respectively (P less than 0.01). The number of monocytes increased from 0.171(+/- 0.113) to 0.501(+/- 0.274) and 0.474(+/- 0.374) x 10(9)/l after 1 and 4 weeks of treatment (P less than 0.01). Other haematologic parameters did not change significantly. Two weeks post-treatment the numbers of neutrophils and monocytes had returned to pre-treatment values. Mild side effects consisting of bone, joint or muscle pain were observed in three patients. Two patients (both with AIDS) did not complete the study. One patient stopped treatment because of fever and malaise, attributable to a generalized cytomegalovirus (CMV) infection and one patient had to stop zidovudine treatment because of severe thrombocytopenia. We conclude that G-CSF increases the number of circulating neutrophilic granulocytes in zidovudine-treated patients at relatively low doses and with few side-effects.