In patients with sleep apnea syndrome a high night by night variability of the respiratory breathing disturbance was reported in some studies. In the following study the variability of the apnoe-/hypopnoeindex (AHI) was prospectively determined with an ambulant monitoring system. Outpatients were instructed to perform three measurement at home independently. The influence of the variability was determined with respect to a first night effect (FNE), body position, severity of SAS, age and sex of the patients. 19 patients (12 men and 7 women, mean age 51.6 +/- 12.4 years) were investigated. The measurements were done with the ambulatory device Jaeger pro (ViaSys Comp., Wuerzburg). No statistically significant differences of the AHI and the cardiorespiratory parameters between the 3 nights were found (AHI 9.8 +/- 11.7/h, 10.1 +/- 9.6/h vs. 8.5 +/- 9.0/h). However the individual AHI difference was 5,6 +/- 5,0/h. In 15 % of the patients the AHI varied even by more than 10 per h. The variability of the AHI was associated with the severity. No FNE, dependency of body position, sex or age was found. Due to the individual variability of the AHI severity in patients with sleep apnoe syndrome, the severity is underestimated in a considerable number of patients. Repetitive measurements improve the diagnostic power.