Objective To study the effect of obstructive sleep apnea syndrome (OSAS) on blood glucose in elderly patients with diabetes mellitus. Methods Eighty elderly diabetic patients were monitored by RS2611 mattress monitoring system of sleep. The data of sleep disordered breathing and relationship between OSAS and blood glucose were analyzed. Results Compared with the non-OSAS patients, all the OSAS patients showed increased fasting and 2-hour postprandial plasma glucose, HbA₁c, CRP and homocysteine. The increments were statistically significantly different in severe OSAS patients and were correlated with different parameters of breathing function during sleep. Conclusions OSAS may produce harmful effect on blood glucose in elderly patients.