The aim of the study was to investigate endothelium-dependent vasodilator responses in the forearm vasculature of patients with systemic vasculitis. We studied 10 patients with systemic vasculitis and 16 healthy control subjects using forearm venous occlusion plethysmography to assess changes in forearm blood flow in response to acetylcholine (ACh). NG-monomethyl-L-arginine (L-NMMA), was also used to assess the contribution of endothelium derived relaxing factor/nitric oxide (EDRF/NO) to the vasodilator responses to acetylcholine. A significantly greater vasodilator response to ACh was seen in the patient group at all doses infused (p < 0.01). After pre-infusion of L-NMMA, the differences in ACh dilator responses were no longer significant. There was a greater magnitude of inhibition of ACh responses by L-NMMA in the patient group. The enhanced vasodilator response to ACh observed was in part abolished by L-NMMA, suggesting that EDRF/NO is produced in excess in systemic vasculitis. The precise role played by EDRF/NO in the pathogenesis of systemic vasculitis requires further study.