Treatment with gamma-interferon (IFN-) is associated with reduced frequency and severity of infections in chronic granulomatous disease (CGD), but the mechanism is unknown. Since the inducible nitric oxide (NO) synthase can be amplified by IFN- in murine macrophages, for example, we hypothesized that IFN- might modulate NO release from polymorphonuclear neutrophils (PMNs) in patients with CGD. Eight patients with CGD and eight healthy controls were studied. Each patient was given either 50 or 100 µg of IFN- per m² on two consecutive days. The production of NO from N-formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated PMNs was assessed as the N^G-monomethyl-L-arginine-inhibitable oxidation of oxyhemoglobin to methemoglobin in the presence of catalase and superoxide dismutase. Prior to IFN- treatment, the PMNs from CGD patients produced 372 ± 27 (mean ± standard error of the mean) pmol of NO/10⁶ PMNs at 45 min, while the control PMNs produced 343 ± 44 pmol. On day 1 after IFN- treatment, NO production increased to 132% ± 25% of that for controls, and on day 3 it reached 360% ± 37% (P < 0.001) of that for controls. On day 8, the values still remained higher, 280% ± 78% more than the control values. Likewise, the bactericidal capacity of PMNs increased on day 3. The present data show that IFN- treatment of CGD patients is associated with an increased production of NO from PMNs when activated by fMLP. Since these PMNs lack the capacity to produce superoxide anions, it is conceivable that this increase in NO release could be instrumental in augmenting host defense.