To investigate whether polycythemia associated with chronic hypoxemic lung disease (CHLD) increases vascular resistance by altering endothelium-derived nitric oxide (NO), we examined the responses to acetylcholine (ACh) infusions (5, 10, and 15 mg/min) on hemodynamics and gas exchange in 21 patients with CHLD of varying severity. Patients were classified into two groups based on whether their hemoglobin (Hb) level was less or greater than 15.5 g/dl. In the normocythemic patients (Hb = 13.6 +/- 0.3 g/100 ml, n = 10), ACh decreased pulmonary artery pressure (Ppa) from 30 +/- 2 mm Hg 26 +/- 2 mm Hg (p < 0.01); pulmonary vascular resistance (PVR), from 5.1 +/- 0.4 U/m2 to 3.4 +/- 0.3 U/m2 (p < 0.001); systemic arterial pressure (Psa), from 111 +/- 4 mm Hg to 108 +/- 4 mm Hg (p < 0.05); and systemic vascular resistance (SVR), from 27 +/- 2 U/m2 to 22 +/- 2 U/m2 (p < 0.01); and also increased the cardiac index (CI), from 3.8 +/- 0.2 to 4.7 +/- 0.3 L/min/m2 (p < 0.001). PaO2 fell from 59 +/- 3 mm Hg to 48 +/- 3 mm Hg (p < 0.001) whereas venous admixture (Qs/Qt) rose from 32 +/- 4% to 44 +/- 4% (p < 0.01). In contrast, in patients with polycythemia (17.7 +/- 0.5 g/100 ml, n = 11) ACh failed to produce any changes in PaO2 (49 +/- 2 mm Hg versus 51 +/- 2 mm Hg, p = NS), Ppa (34 +/- 1 mm Hg versus 33 +/- 1 mm Hg, p = NS), PVR (6.7 +/- 0.9 U/m2 versus 6.9 +/- 0.8 U/m2, p = NS) or Psa, but slightly increased the CI, from 3.6 +/- 0.3 L/min/m2 to 3.9 +/- 0.3 L/min/m2 (p < 0.01), and Qs/Qt, from 40 +/- 4% to 45 +/- 3% (p < 0.05). In the 21 patients, negative correlations with Hb concentrations were found for ACh-induced changes in PVR (r = -0.57, p < 0.01), Ppa (r = -0.46, p < 0.01), CI (r = -0.5, p < 0.05), PaO2 (r = -0.79, p < 0.01), and Qs/Qt (r = -0.79, p < 0.01). In the six polycythemic patients who received isovolemic hemodilution, with a decrease in Hb concentration from 18.6 +/- 0.9 g/dl to 15.3 +/- 0.3 g/dl as a result, infusion of ACh, which was without effect before hemodilution, caused decreases in Ppa from 28 +/- 1 mm Hg to 23 +/- 1 mm Hg (p < 0.05) and in PVR from 5.7 +/- 0.8 U/m2 to 3.6 +/- 0.5 U/m2 (p < 0.02), as well as an increase in CI from 3.4 +/- 0.4 L/min/ m2 to 4.1 +/- 0.4 L/min/m2 (p < 0.05). In contrast to ACh, inhaled NO (40 ppm) induced pulmonary vasodilation in both the normocythemic and polycythemic groups. Our results show that high hematocrit (Hct) levels inhibit endothelium-dependent vasodilation in response to ACh in patients with CHLD, possibly through inactivation of endothelial-derived NO by Hb.