This prospective open trial evaluated the efficacy and tolerability of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) in patients with established neutropenia, considering as the main endpoint the clinical benefit to the patients regarding clearing of infection or resuming chemotherapy as initially planed. Adult patients (n = 28) with absolute neutrophil counts (ANC) < 10(9)/1 for 21 days were given a fixed dose (400 micrograms) of rhGM-CSF subcutaneously, for a total of 35 cycles. Causes of neutropenia were chemotherapy for acute leukaemia, lymphoma, myeloma and solid tumours, complications after bone marrow transplantation (BMT), and neutropenia associated with AIDS. Response (ANC to > 10(9)/l) occurred in 83% of rhGM-CSF cycles (29/35). Median time to response was 2.4 days (mean 6.7 days). Kinetics of response was dependent on diagnosis and treatment history. Fever abated with increasing ANC in 13/17 patients (76%) who entered the trial with hyperpyrexia. Treatment with rhGM-CSF allowed chemotherapy to be resumed on schedule in 7/9 relevant cycles. Toxicity was mild, leading to treatment interruption in only two cycles. In conclusion, rhGM-CSF was well tolerated and associated with a rise in ANC which appeared to result in immediate clinical benefit, including resolution of infection and resumption of scheduled chemotherapy.