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Capnography, the non-invasive monitoring of the CO2 partial pressure in the exhaled gas provides information on lung ventilation and perfusion. The reference mainstream (MS) method measures CO2 directly at the airway opening. Sidestream (SS) devices analyse a gas sample aspirated through a tube, distorting the capnogram in an unclarified manner and degree. We investigated the influence of sampling rate on the shape factors of the SS capnogram. Measurements were made simultaneously with MS and SS capnographs, the latter being equipped with adjustable sampling flow in 6 anaesthetised, ventilated patients. Phase 2 and 3 slopes (S2 and S3) were derived from time capnograms. S2 describes the transition between the emptying of bronchial and alveolar gas, S3 reflects the heterogeneity of alveolar ventilation. To assess the effects of sampling flow on capnogram shape, we analysed correlations between the sampling flow and the difference of SS and MS parameters (S2diff, S3diff). By increasing sampling flow from 100 to 400 ml/min in 3 steps, S2diff exhibited monotonous, linear decrease (R2=0.47, p Our results highlight that changes in sampling flow rate bias the shape factors derived from SS capnogram. Increasing the sampling flow from 200 to 400-500 ml/min improves the accuracy of SS capnographic parameter estimations, approaching those obtained by the MS method. In clinical practice, however, the feasibility of increasing the sampling flow is limited by the loss of tidal volume, which is of great importance in paediatric patients. Supported by OTKA grant K115253.