The prevalence of sleep apnea was studied in 46 middle- and older-aged men with "essential hypertension." Thirty-four age- and weight-similar normotensive men were also studied. Fourteen hypertensive men and three controls had sleep apnea syndrome, as defined as greater than ten apneas per hour of sleep. Hypertensive men with apnea tended to be more overweight and slightly older than the hypertensive men without apnea, but differences were not statistically significant. Individual men with apnea could not be distinguished by their answers on a questionnaire that elicited symptoms related to apnea. Seven hypertensive men with apnea were treated with protriptyline and one with uvulopalatopharyngoplasty, and apnea index (apneas per hour) decreased by 77% from pretreatment levels while mean blood pressure decreased from 149/95 mm Hg to 139/90 mm Hg. Undiagnosed sleep apnea syndrome may be associated with systemic hypertension in many middle- and older-aged men. In some, sleep apnea syndrome could be the cause of hypertension, and in others it may contribute to hypertension of another cause.