Study Objectives: To determine prevalence of depressive symptoms in obstructive sleep apnea (OSA) and the impact of OSA treatment on depression scores. Methods: Consecutive new patients referred for investigation of suspected OSA were approached. Consenting patients completed a patient health questionnaire (PHQ-9) for depressive symptoms when attending for laboratory polysomnography. Those with moderate/severe (apneahypopnea index [AHI] = 15 events/h) and/or symptomatic mild OSA (AHI 5-14.99 events/h) were offered continuous positive airway pressure (CPAP) therapy. PHQ-9 was repeated after 3 months of CPAP with compliance recorded. Of a maximum PHQ-9 score of 27, a cut point = 10 (PHQ-9 ≥ 10) was used to indicate presence of clinically signifi cant depressive symptoms. Results: A total of 426 participants (243 males) were recruited. Mean ± standard deviation body mass index (BMI) was 32.1 ± 7.1 kg/m2 and AHI 33.6 ± 28.9 events/h. PHQ-9 was 10.5 ± 6.1 and independently related to AHI (p <0.001) and BMI (p <0.001). In those without OSA, PHQ-9 ≥ 10 was more common in women, but no gender difference was evident with OSA. Of 293 patients offered CPAP, 228 were compliant (mean nightly use > 5 h) over 3 months of therapy. In them, with therapy, AHI decreased from 46.7 ± 27.4 to 6.5 ± 1.6 events/h, PHQ-9 from 11.3 ± 6.1 to 3.7 ± 2.9 and PHQ-9 = 10 from 74.6% to 3.9% (p <0.001 in each case). Magnitude of change in PHQ-9 was similar in men and women. Antidepressant use was constant throughout. Conclusions: Depressive symptoms are common in OSA and related to its severity. They improve markedly with CPAP, implying a relationship to untreated OSA.