Obesity-related sleep apnea syndrome (SAS) was diagnosed in 13 patients evaluated for gastric bypass surgery. A diagnostic sleep study was performed whenever a specially designed questionnaire revealed characteristic signs of sleep disturbances. Pretreatment polyhypnographic recordings of patients with SAS demonstrated considerable reduction of deep and rapid eye movement (REM) sleep stages with a correspondent prolongation of wake within sleep or non-REM sleep stages I and II. After surgical weight reduction repeated polyhypnographic recordings revealed considerable improvement or even a complete recovery of breathing in sleep and a normalization of sleep structure. Non-REM deep sleep stages (III and IV) augmented from 5.51% +/- 2.53% (mean + SEM) to 22.69% +/- 3.56% (p less than 0.002), and the REM stage increased from 9.91% +/- 1.78% to 18.15% +/- 2.13% (p less than 0.005). Surgical weight reduction in obesity-related SAS is a valuable therapeutic measure for this respiratory derangement, as well as for sleep quality.