Increased activity of inducible nitric oxide synthase (NOS) has been found in cardiac tissue and in skeletal muscle from patients with chronic congestive heart failure (CHF). There have been few reports investigating NOS activity in other organs or in peripheral blood cells from patients with chronic CHF. To examine whether NOS activities in peripheral polymorphonuclear leukocytes (PML) are increased in patients with chronic CHF and to determine whether a correlation exists between disease severity and NOS activity in PML of patients with chronic CHF, we assessed the levels of NOS activity in PML by measuring the capacity of isolated PML to convert ³H-L-arginine to ³H-L-citrullin in 70 Japanese patients with chronic CHF and in 24 age-matched healthy volunteers. The levels of NOS activity in PML were significantly greater in patients with chronic CHF than in healthy volunteers (18.0 ± 0.6% vs 11.5 ± 0.3%, p <0.01). NOS activity in PML was increased with the severity of New York Heart Association functional class. Among the various neurohumonal factors, the plasma levels of interleukin-6, atrial natriuretic peptide, and norepinephrine showed independent and significant positive relations with levels of NOS activity in PML. Thus, we demonstrated that NOS activity in PML was elevated in patients with chronic CHF in relation to the severity of heart failure, circulating proinflammatory cytokines, and neurohormonal factors.