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Background: The measurement of mouth occlusion pressure is an easy method to assess respiratory drive and muscle function. Lately there is a variety of computer assisted programmes, but their precision is discussed controversially. In this study we analysed the repeatability of measurements of mouth occlusion pressures performed by a computer assisted measurement unit., Patients and Methods: We examined maximal inspiratory mouth occlusion pressure (PI(max)), mouth occlusion pressure 100 ms after onset of inspiration (PO.1) and PI(max) after 100 ms (PO.1(max)) of 32 patients with COPD. Subjects were divided into group A (normal blood gas pressures) and group B (hypercapnia due to alveolar hypoventilation). Controls (K) consisted of 22 healthy men. Criteria for repeatability were analysis of variance, intraindividual standard deviation of the mean and the variation coefficient of 5 consecutive measurements., Results: Controls produced the highest values for PI(max) and the lowest ones for PO, 1. Intraindividual standard deviation was small as well as the variation coefficient; we found no statistically significant differences. Compared with controls in group A PI(max) values were markedly reduced with elevated PO.1 values. Intraindividual standard deviation and variation coefficient were low, differences not significant. We observed similar results for group B, variance analysis showed no significant differences, deviation around the mean was small., Conclusion: Our data suggest a high repeatability of measurements of mouth occlusion pressures. Especially patients with severe COPD and alveolar hyperventilation are able to produce valid and reliable test results.