A case of severe obstructive sleep apnea developing during pregnancy is reported. A 27-year-old primigravida was well until the sixth month of pregnancy, when she developed loud snoring and excessive daytime sleepiness. Polysomnography was performed at 36 weeks' gestation and revealed severe obstructive sleep apnea. The patient was treated successfully during pregnancy with nasal continuous positive airway pressure, but continued to suffer from moderate obstructive sleep apnea after delivery. This case suggests that sleep apnea may be either precipitated or exacerbated during pregnancy.