In order to study the possible relationship between sleep apnea syndrome (SAS) and diabetes mellitus, we first examined the prevalence of SAS among 12,787 general patients (6554 males and 6233 females) who visited Katsumata Hospital at Nagoya, Japan. Among them, thirty-five males and five females were diagnosed as having SAS. The male patients were statistically analysed by the corrected Mantel-Haenszel chi-square test taking the body type into account, and it was found that the prevalence of SAS was significantly high both in a diabetic population and in a hypertensive one. Among 40 SAS patients of both sexes, 34 were given a glucose tolerance test (GTT) with oral administration of 75 g glucose. Thirteen showed a diabetic pattern, 12 a borderline pattern and only 9 had a normal pattern. All 13 diabetic patients had non-insulin-dependent type diabetes (NIDDM). The present results showed that SAS has a close relationship not only to hypertension but also to NIDDM.