We performed overnight polysomnography in 77 patients for diagnosis of sleep apnea syndrome (SAS) and assessment of severity. Patients were classified into 3 groups according to apnea index (AI): group 1 (G1; no SAS), AI < or = 10 (21 patients), group 2 (G2; moderate SAS), 10 < AI < 40 (34 patients), and group 3 (G3; severe SAS), AI > or = 40 (22 patients). In all 3 groups, 60% of the apneas were obstructive, but the proportion of mixed apnea increased from G1 to G3 (6% in G1, 12% in G2, and 24% in G3). The duration of REM sleep was shorter in G2 and G3 than in G1 (G1 61 min, G2 44 min, G3 41 min; p < 0.03). In G3 a dramatic decrease in deep sleep was noted in comparison with G1 and G2 (G1 29 min, G2 31 min, G3 10 min). The distribution of the apnea in the different sleep stages was similar in G3; in G2, 16 patients had a REM apnea predominance, but in this subgroup duration of REM sleep was longer and AI was lower than in the other 18 G2 patients. So, absence of sleep stage apnea predominance and high proportion of mixed apnea were observed in the most severe SAS.