We reviewed clinical data, autopsy reports, and microscopic slides on 10 patients with sleep apnea/obesity hypoventilation syndrome (SA/OHS) to define the cardiopulmonary pathological features and establish clinicopathologic correlations. Ten obese (>136 kg) patients without SA/OHS were studied as controls. Patients with SA/OHS exhibited biventricular cardiac failure and pulmonary hypertension with a higher prevalence of moderate/severe pulmonary hemosiderosis (8 v 0 patients), alveolar hemorrhage (7 v 4 patients), capillary proliferation (4 v 0 patients), iron encrustation of elastica (1 v 0 patients) and medial hypertrophy of muscular pulmonary arteries (11.9 +/- 2.4 v 9.7 +/- 1.6%) (P < .05). In two patients capillary proliferation resembled capillary hemangiomatosis. Mean right ventricular thickness was higher in the SA/OHS group (0.71 +/- 0.17 v 0.42 +/- 0.1 cm) (P < .01). Four patients with SA/OHS and three controls had moderate/severe myocardial fibrosis. Biventricular cardiac failure caused death in seven patients with SA/OHS. Hypoxia is probably the most important cause of pulmonary hypertension, arterial muscularization, and right ventricular hypertrophy in SA/ OHS. Left ventricular failure in some SA/OHS patients may be the result of hypertensive cardiac disease. In others, the etiology of left ventricular failure was not determined morphologically, suggesting functional abnormalities related to obesity and/or apneic episodes.