In the present report we investigated whether oscillometric and plethysmographic arterial blood pressure measurement techniques yielded different results compared to invasive blood pressure measurements in 18 mechanically ventilated and 14 spontaneously breathing patients. METHODS. Blood pressure was recorded simultaneously with plethysmographic, oscillometric, and invasive systems (FINAPRES 2300, HP 78352A, and 78534A, respectively). Invasive measurements were obtained in the radial artery. Short tubing was chosen in order to avoid transmission errors. The finger cuff of the FINAPRES 2300 was placed on the ipsilateral middle phalanx of the middle finger. The cuff of the oscillometric system was located on the contralateral arm. Differences in arterial blood pressure had been excluded. RESULTS. Histograms of the differences in the various pressure measurements, linear regression, and correlation coefficients were determined for quantitative comparison. The two non-invasive measurement devices (FINAPRES 2300, HP 78352A) yielded similar results in spontaneously breathing patients. In ventilated patients the reliability of oscillometric measurements was worse than that of the photoplethysmographic, especially for diastolic and mean arterial blood pressure values (Table 1, Figs. 2-4). DISCUSSION. The reasons for the differences between invasive and non-invasive measurement techniques are most likely due to problems with cuff handling for the plethysmographic device and the principles of oscillometric measurement. Both methods have to be optimised in hardware and software. However, the photoplethysmographic results showed an improvement in blood pressure measurement in ventilated patients. Due to the remaining deviations between the non-invasive and invasive measurements, especially in critically ill patients in the intensive care unit, direct measurement cannot be replaced by either of the non-invasive methods.