Patient histories suggest that obstructive sleep apnea syndrome (OSAS) is a progressive condition. To investigate whether this could be shown in data from sleep recordings, 42 patients with OSAS were retrospectively studied. All had undergone a screening recording of respiration movements and oximetry at least 6 months (average, 16 months) prior to a diagnostic polysomnogram including these parameters. No treatment was given in the meantime. In the first recording, mean oxygen desaturation index (ODI, average number of desaturations of > 4 percent per sleeping hour) was 10; periodic, obstructive respiration movements occurred during (average) 36 percent of total estimated sleeping time, and mean nadir SaO2 was 85 percent. In the second recording, mean ODI was 21 (significant change, p = 0.0002), periodic respiration time was 61 percent, and nadir SaO2 was 80 percent (p = 0.0001, respectively). In 26 of 42 patients (62 percent), ODI had increased by > 50 percent. Increases in ODIs and periodic breathing were significantly correlated to increases in body weight. There were, however, exceptional patients with considerable increases in respiratory disturbance despite weight loss. The greatest changes were found in the patients who had the highest apnea indices in the polysomnograms. Early treatment may therefore be justified, since a borderline case may change to severe OSAS in 1 to 2 years' time. Follow-up recordings of untreated patients are important.