Fifty-five patients with obstructive sleep apnea each completed a Zung Self-Rating Depression Scale (SDS), Twenty-five patients (45%) had SDS scores greater than or equal to 50, consistent with depression. The SDS scores did not correlate with age, the number of respiratory events per hour sleep, antihypertensive medication, or the oxygen saturation baseline or nadir. The group with SDS scores of 50 or greater, however, had 68.0 +/- 8.8 respiratory events per hour compared with 47.9 +/- 4.7 in the group with SDS scores under 50 (p less than .05). Nineteen patients who were treated with nasal continuous positive airway pressure completed a follow-up SDS Inventory. After treatment, the SDS scores fell from 60.5 +/- 1.9 to 44.4 +/- 2.6 (p less than .001) in the 11 patients with baseline elevated scores. The authors conclude that obstructive sleep apnea can produce prominent symptoms of depression that appear to be related to the severity of the underlying apnea; furthermore, treatment of obstructive sleep apnea may result in alleviation of these symptoms in certain patients.