The prevalence of sleep apnea syndrome (SAS) in Japan has been reported to be about 1-2%, which is almost the same as that reported in western countries. However, an epidermiological survey of SAS recently performed by institutes participating in the Japanese Society of Sleep-Disordered Breathing showed that only 1,425 patients were newly diagnosed in 1993, which suggests that there are many latent or undiagnosed cases in Japan, because SAS is newly diagnosed in 75,000 patients every year in the USA. Here we reported on issues related to the diagnosis and treatment of SAS in Japan and in other countries.