Rationale: Chronic mountain sickness or Monge's disease is characterized by an excessive polycythemia in high-altitude dwellers, with a prevalence of 5 to 18% above 3,200 m. To date, no pharmacologic treatment is available. Objectives: We evaluated the efficacy of acetazolamide in the treatment of chronic mountain sickness and the importance of nocturnal hypoxemia in its pathophysiology. Methods: A double-blind placebo-controlled study was performed in three groups of patients from Cerro de Pasco, Peru (4,300 m), treated orally for 3 weeks with placebo (n = 10), 250 mg of acetazolamide (n = 10), or 500 mg of acetazolamide (n = 10), daily. Results: Acetazolamide decreased hematocrit by 7.1% (p < 0.001) and 6.7% (p < 0.001), serum erythropoietin by 67% (p < 0.01) and 50% (p < 0.001), and serum soluble transferrin receptors by 11.1% (p < 0.05) and 3.4% (p < 0.001), and increased serum ferritin by 540% (p < 0.001) and 134% (p < 0.001), for groups treated with 250 and 500 mg of acetazolamide, respectively. Acetazolamide (250 mg) increased nocturnal arterial O2 saturation by 5% (p < 0.01) and decreased mean nocturnal heart rate by 11% (p < 0.05) and the number of apnea-hypopnea episodes during sleep by 74% (p < 0.05). The decrease in erythropoietin was attributed mainly to the acetazolamide-induced increase in ventilation and arterial O2 saturation. Conclusions: Acetazolamide, the first efficient pharmacologic treatment of chronic mountain sickness without adverse effects, reduces hypoventilation, which may be accentuated during sleep, and blunts erythropoiesis. Its low cost may allow wide development with a considerable positive impact on public health in high-altitude regions.