The prevalence and mechanisms of daytime pulmonary hypertension were examined in 24 cases of obstructive sleep apnea syndrome (OSAS). All patients were free from chronic lung disease. They underwent pulmonary function tests and blood gas analysis in the sitting and supine position, hypercapnic ventilatory response test, exercise test and right heart catheterization. Elevation of mean pulmonary arterial pressure (m-PAP) above 20 mmHg was observed in 5 out of 24 cases (20.8%). The group with pulmonary hypertension (PG+: m-PAP = 22.2 +/- 2.7 mmHg) showed marked obesity (p < 0.001), significant decrease of supine FRC/TLC (p < 0.05), increase of supine CC/FRC (p < 0.01), decrease of supine PaO2 (p < 0.02) and desaturation during exercise (p < 0.05) in comparison with the group without pulmonary hypertension (PH-: 13.9 +/- 3.1 mmHg). m-PAP was positively correlated with %IBW and desaturation during exercise (p < 0.01, p < 0.02) and negatively correlated with supine PaO2 (p < 0.01). Various changes in pulmonary function and pulmonary hemodynamics due to obesity seem to lead to daytime pulmonary hypertension of OSAS.