To a great extent, the risks of autologous bone marrow transplantation are related to neutropenia. Although the efficacy of the recombinant human granulocyte-macrophage colony-stimulating factor (rhu GM-CSF) on neutrophil recovery has appeared in numerous open trials, only a few randomized studies have hitherto been published. Ninety-one patients with non-Hodgkin's malignant lymphoma treated with ablative chemotherapy followed by purged or unpurged bone marrow transplantation were entered in a placebo-controlled, double-blind randomized study; 44 patients received GM-CSF (E. coli) in doses of 250 micrograms/m2/day, and 47 received a placebo. Treatment was administered daily as continuous infusion started on the day of transplantation and pursued until the absolute number of neutrophils reached 0.5 x 10(9)/l during 7 days or, if this failed, during 30 days. The median time of neutrophil recovery was 14 days in patients on rhu GM-CSF and 21 days in patients on placebo (P < 0.0001). Patients who received a mafosfamide-purged bone marrow also had a rapid neutrophil recovery (median: 16 days versus 20.5 days; P = 0.013). The stay in hospital was shorter in the rhu GM-CSF group (median: 23 days versus 28 days; P < 0.05). No significant difference in the number of days with fever, infections, antibiotics administered and overall survival was detected between the two groups. The main toxicity ascribable to rhu GM-CSF was a capillary leakage syndrome found in 3 patients. Thus, after purged or unpurged autologous bone marrow transplantation rhu GM-CSF significantly reduces the duration of both neutropenia and hospital stay.