A validation study was performed on the MESAM 4, a digital recording device developed to monitor oxygen saturation, heart rate (HR), snoring, and body position in order to screen subjects for obstructive sleep apnea syndrome (OSAS). MESAM 4 recordings were scored with the computer-based automatic scoring system provided with the equipment. Nocturnal polysomnography (PSG) and MESAM 4 recordings were run simultaneously on 56 subjects presenting with any type of sleep complaint, including those secondary to OSAS. Patients were assigned to one room by hospital administration and were monitored consecutively. The polygraphic equipment and MESAM 4 equipment were placed on the subjects by separate teams. Records of PSG and MESAM 4 were analyzed in double-blind fashion. With the MESAM 4 computerized analysis, three indices based on SaO2 (ODI), on heart rate (HVI), and on snoring (ISI) were obtained, and the number of abnormal respiratory events occurring during the time selected for analysis (TAT) were determined. Polysomnographic records were scored by 30-s epochs following the American Sleep Disorders Association standards for sleep states and stages and for sleep-related events, including sleep apneas, hypopneas, and periodic leg movements. Following independent scoring, 26 subjects were identified with OSAS by PSG, while MESAM 4 identified 25 subjects with OSAS using oxygen algorithm; all had a respiratory disturbance index greater than or equal to 10 with PSG. Results of each polysomnogram and each MESAM 4 analysis were compared. With the polysomnogram used as a standard, the degree of error for each variable with the MESAM 4 was calculated. Specificity and sensitivity of the most accurate index of the MESAM 4, the ODI, were 97 percent and 92 percent, respectively. The other two indices, HVI and ISI, were less accurate: specificity and sensitivity were 32 percent and 58 percent for HVI and 27 percent and 96 percent for ISI. Nevertheless, a combination of all three indices (ODI, HVI, ISI) would have prevented the two false-positive cases we observed. The results of this validation study show that MESAM 4 can be helpful to general practitioners, clinicians, and epidemiologists as a low-cost screening device for subjects with OSAS and habitual snoring.