The development of inexpensive tools for diagnosing sleep apnea syndrome (SAS) is a result of the high prevalence of this condition and of the high cost of polysomnograms (PS). MESAM IV is a portable device that records changes in oxygen saturation (SO2), heart rate (HR) and snoring (S). Readings can be automatic or manual, the latter in function of an events index (EI), with a graph of the three variables generated. We carried out a simultaneous study of 51 subjects suspected of having SAS who were referred to the sleep unit by the pneumology outpatient clinic. PS was interpreted manually at 30-sec intervals as recommended by the American Thoracic Society. An apnea/hypoapnea index (AHI) > or = 10/hour of sleep was used as the cutoff point for SAS. Thirty-two (63%) subjects were found to have SAS as indicated by PS. The rate of agreement between AHI and automatic analysis of SO2, HR and S was only moderate (intra-group correlation coefficients -ICC- of 0.50, 0.40, and 0.53, respectively) and was inferior to manual analysis with EI (ICC of 0.77). Assessment of diagnostic efficacy of automatic analysis in terms of sensitivity (SEN), specificity (SPE), positive predictive value (PPV) and negative predictive value (NPV) yielded the following results: SO2 (SEN 94%, SPE 26%, PPV 68% and NPV 71%), HR (SEN 59%, SPE 58%, PPV 70%, NPV 46%); S (SEN 84%, SPE 26%, PPV 66%, NPV 50%). Manual analysis (EI) gave more valid results (SEN 100%, SPE 84%, PPV 91%, NPV 100%). If patients with chronic obstructive lung disease are excluded, however, the results for automatic analysis improve: SEN 100%, SPE 91%, PPV 96%, NPV 100%. These results show that MESAM IV is of great help in diagnosing SAS, allowing better screening for identifying candidates for PS.