Hypoxemia during hemodialysis has variously been attributed to worsening ventilation-perfusion (VA/Q) relationships, alveolar hypoventilation combined with a reduced respiratory quotient, increased right-to-left shunting, and diffusion impairment. It is difficult to separate out these various effects, which explains lack of agreement in the literature. To more critically evaluate the causes of hypoxemia during hemodialysis, we used a multiple inert gas elimination technique to determine the distribution of ventilation-perfusion ratios during hemodialysis in 8 patients with chronic renal failure. Measurements were made before, during (at 60, 120, and 210 min), and after hemodialysis. Whereas arterial PO2 fell from 87 to 74 mmHg by 120 min, ventilation-perfusion relationships actually improved. Cardiac output fell from 5.3 to 4.0 L/min over the same time. Alveolar ventilation, respiratory quotient, and alveolar PO2 all fell, and the alveolar arterial PO2 difference remained essentially unchanged. These findings suggest that the hypoxemia observed during hemodialysis is primarily due to a decrease in alveolar ventilation and respiratory quotient associated with removal of metabolic CO2 in the dialyzer. Secondary factors affecting arterial PO2 were the slight improvement in ventilation-perfusion relationships tending to increase it, and the decrease in cardiac output tending to decrease it. There was no evidence for diffusion impairment because the measured VA/Q inequality accounted for the degree of hypoxemia.