Chronic mountain sickness is a syndrome of severe symptomatic polycythemia and hypoxemia occurring in natives or long-term high altitude sojourners. The condition may be complicated by pulmonary hypertension in proportion to decreased oxygenation, indicating hypoxic vasoconstriction and remodeling. Exercise in these patients is associated with a steep slope of pulmonary artery pressure-flow relationships and decreased vascular distensibility. Correction of pulmonary vascular resistance for increased hematocrit decreases the severity of pulmonary hypertension. Exercise-induced pulmonary hypertension in chronic mountain sickness does not affect exercise capacity, in relation to high oxygen content of the blood and increased lung diffusing capacity. Right ventricular failure seems to be an uncommon complication of chronic mountain sickness, but the exact prevalence of the condition is not known. Acetazolamide given for 6 months to patients with chronic mountain sickness improves oxygenation, polycythemia, and pulmonary artery pressure.