Polysomnographic recordings of 43 children and adults with Prader-Willi syndrome (PWS) were inspected and classified into 5 age groups. The effect of age and body mass index (BMI) on measures of breathing, oxygen saturation, and sleep efficiency were analyzed. Body mass index (BMI) increased significantly between early childhood and preadolescent groups. Subjecting the data to analysis of variance showed an overall significant effect of BMI but no age effect on breathing parameters and oxygen saturation. Increased BMI was associated with decreased oxygen saturation and with higher apnea/hypopnea index. Sleep efficiency index was significantly lower in adults than in young children, preadolescent, and adolescent groups. These findings emphasize the role of obesity in the development of sleep-related breathing abnormalities and nocturnal oxygen desaturation in patients with PWS.