Twelve patients with chronic obstructive pulmonary disease (COPD) were studied in order to evaluate the effect of chlormadinone acetate (CMA), a potent synthetic progesterone, on the degree of hypoxemia during sleep. In patients designated as "correctors," in whom an increase in minute ventilation (VI) during wakefulness was brought about mainly by an increase in tidal volume (VT) and PaCO2 was effectively decreased by the administration of CMA, this agent also proved to be effective during sleep, with hypoxemia improved during both NREM and REM sleep. On the other hand, in patients called "noncorrectors" in whom a decrease in PaCO2 was not seen during wakefulness with CMA, there was also no effect during sleep. These results indicate that CMA is effective in certain selected patients with COPD.