Sleep apnea can lead to marked psychological distress including some mood symptoms. Previous studies on continuous positive airway pressure (CPAP) treatment usually reported significant improvement in mood symptoms in patients with sleep apnea, but most of them did not consider the placebo effect of CPAP. We examined the effect of CPAP treatment on mood states by employing both CPAP treatment and placebo CPAP. Twenty five men and nine women with sleep apnea underwent two successive nights of polysomnography (PSG) during hospitalization. The Profile of Mood States (POMS) was administered to measure mood states. Patients were randomly assigned to either CPAP treatment group or placebo CPAP group. After 7 days of CPAP use at home, all patients were re-hospitalized to undergo one more night of nocturnal PSG with their assigned treatment (CPAP treatment or placebo CPAP). They also had a repeat evaluation of mood by completing the POMS. Only patients on CPAP treatment improved significantly in apnea index, respiratory disturbance index, and mean oxygen saturation. However, both CPAP treatment group and placebo CPAP group showed significant improvement in mood states. In conclusion, the effect of CPAP treatment on mood symptoms in apneic patients could be a placebo effect. CPAP treatment may be effective in improving mood states only in patients who have severe depressive symptoms secondary to sleep apnea.