The prevalence of the sleep apnea syndrome (SAS) among Swedish men 30-69 years old was estimated by a two-stage procedure. In the first stage, 4064 questionnaires were mailed to a random sample of a defined population in the municipality of Uppsala. The response rate was almost 80%; 15.6% of the responders were habitual snorers and 5.8% complained of daytime sleepiness. From these, a group of 166 men highly suspected of having SAS was selected. Eventually, 61 of these came for all-night polysomnographic studies, and 15 of these were found to have SAS. On this basis the lower limit of the prevalence of SAS was estimated to be as high as 1.3%. The majority of subjects with the syndrome were in the age group 50-59 years.