The purpose of this study was to investigate the prevalence of sleep apnea syndrome (SAS) among patients with essential hypertension. Sixteen of 50 patients with essential hypertension were suspected of having SAS based on their responses to a sleep questionnaire. Whole-night polysomnographic recordings revealed that 11 of the 16 patients, which is 22% of the initial sample, had SAS as defined by the occurrence of at least 10 apneas lasting 10 seconds each per hour of polygraphically defined sleep. The preponderant finding of SAS in this group indicates that the possibility of the syndrome should be taken into consideration in the clinical evaluation of these patients.