More than half of patients with essential hypertension have sleep apnea. The incidence of unrecognized sleep apnea in patients with essential hypertension was assessed. Twenty-three patients taking antihypertensive medication were selected at random from a hypertension clinic. They were evaluated by questionnaire for symptoms of sleep apnea, and during 3 hours of sleep, measurements were made of respiratory patterns using an impedance pneumograph, arterial O2 saturation with an ear oximeter and air flow at the mouth or nose with a face mask pneumotacograph. Abnormal sleep apneas (average 20 seconds) lasting for an average of 19% sleep time were found in 11 patients (48%). Significant arterial O2 desaturation, defined as a decrease of at least 4% and to less than 90%, was observed in 7 of these 11 (30%), with an average saturation of 87% at the end of the apneic episodes. Thus, almost one-third of patients randomly selected had significant arterial O2 desaturation during sleep because of sleep apnea, and it is suggested that sleep apnea may play a part in the development of essential hypertension.