Some patients with chronic obstructive pulmonary disease (COPD) experience transient arterial hypoxemia (TAH) during rapid eye movement (REM) sleep. To examine the effect of short- and long-term low flow oxygen on TAH associated cardiopulmonary hemodynamics, we recorded pulmonary artery pressure (Ppa) and cardiac output during nocturnal sleep in seven male subjects with COPD. In all of the subjects, parameters were measured breathing room air at baseline time and after eight or more weeks of home supplemental oxygen (15 hours per day, 3 L/min). Five were also studied one full night at baseline time while breathing 3 L/min nasal oxygen. While breathing room air both before and after chronic home oxygen therapy, transient increases in Ppa during TAH were due to increased pulmonary vascular resistance in seven instances, increased cardiac output in four, and increases in both vascular resistance and cardiac output in two. Short-term supplemental oxygen lowered mean sleeping Ppa and eliminated TAH along with its associated hemodynamic changes in four of the five subjects; the fifth did not experience REM sleep. In the six subjects who complied with the eight-week home oxygen protocol, mean sleeping Ppa was significantly reduced (p less than 0.05). In four of these, total pulmonary resistance was lower and cardiac output higher after home oxygen therapy. Short-term supplemental oxygen is useful in correcting REM-associated TAH and in some hypoxemic subjects, reducing mean sleeping Ppa. Sustained reductions in pulmonary vascular resistance after long-term home oxygen therapy may be indicative of improved cardiac and pulmonary vascular status even in subjects showing minimal or no reduction in mean sleeping Ppa.