Fourteen patients with chronic obstructive pulmonary disease (COPD) and chronic hypoxemia were studied to evaluate the relationship between hypoxemia and objective and subjective daytime sleepiness. Patients were selected with a waking PaO2 of less than 70 mm Hg and less than 50 percent predicted FEV1. Clinically, none of these patients had complaints of significant daytime sleepiness. Each patient underwent standard all-night polysomnographic evaluation followed by a multiple sleep latency test (MSLT). There was no significant correlation between the mean sleep onset latency for the MSLT and the waking PaO2, PCO2, FEV1, or spontaneous desaturations during sleep. Patients with COPD exhibited a mean MSLT that was within normal limits, despite a short total sleep time and numerous arousals from sleep. We conclude that there appears to be no relationship between chronic daytime hypoxemia and subjective reports and objective measures of daytime sleepiness.