We evaluated recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF; Sandoz Pharma [Basel, Switzerland]/Schering-Plough [Kenilworth, NJ]) as an adjunct to a modified (mainly cyclophosphamide and doxorubicin increased 1.5-fold) COP-BLAM regimen in the primary treatment of high-grade malignant non-Hodgkin's lymphomas (NHL). Patients (n = 182; stage II-IV; age, 15 to 73 years) were randomized to rhGM-CSF (400 micrograms) or placebo for 7 days subcutaneously after chemotherapy. Efficacy was analyzed for patients receiving at least 70% of study medication (n = 125). The frequency of clinically relevant infection was reduced by rhGM-CSF (28 v 69 infections, 16 v 30 patients, P = .02) with a cumulative probability of remaining infection free in 70% versus 48% (P = .05 log rank test at 190 days). Periods of neutropenia (P = .01 in 5 of 6 courses), days with fever (2.1 v 4.0, P = .04) and days of hospitalization for infection (3.5 v 8.0 days, P = .01) were significantly reduced. Complete response (CR) rates, assessed by prognostic risk, were 15 of 19 (79%) in treated versus 20 of 21 (95%) in controls in the low-risk group (P = .12). In the high-risk group, 31 of 45 (69%) treated patients achieved CR versus 25 of 52 (48%) of controls (P = .04). No difference in survival has been seen after 1 year. Only injection site reactions (45% treated v 7% controls) and rash (26% v 2%) occurred more frequently in treated patients (n = 176). These data show that rhGM-CSF is well tolerated in most patients with NHL, significantly reduces infection, and improves response.