Nonapneic, nocturnal oxyhemoglobin desaturation (NOD) during rapid-eye-movement (REM) sleep has been proposed as a predecessor and possible etiologic factor in the development of pulmonary hypertension in patients both with restrictive and obstructive chronic lung disease. The association between abnormal waking cardiopulmonary hemodynamics and NOD has not been established in patients with mild daytime hypoxemia. Gas exchange, pulmonary function, red cell mass, radionuclide gated cardiac ejection fraction, and supine cardiopulmonary hemodynamics were examined in 36 patients with chronic lung disease. All had a daytime PaO2 greater than 60 mm Hg and REM sleep-related NOD for greater than 5 minutes, to 85 percent or lower. These data were compared to those from 13 subjects with similar symptoms and objective measures of pulmonary dysfunction but without evidence of NOD. Patients with NOD showed more end organ evidence of hypoxemia and more abnormal cardiopulmonary hemodynamics than patients with similar degrees of lung disease but without NOD. The relative role of daytime vs nocturnal hypoxemia in inducing hemodynamic differences between groups cannot be determined from this study.