OBJECTIVE This study was undertaken to assess the clinical usefulness of a continuous intra-arterial blood gas (CIABG) monitoring system, Paratrend 7, during thoracic surgery. METHODS A sensor of the CIABG monitoring system was inserted into the radial artery in 50 patients. During one-lung/differential lung ventilation, arterial blood samples for estimation of blood gases (ABG) were taken every 45-90 min or when clinically needed. The PCO(2), PO(2), and pH values displayed by the CIABG monitor were recorded prior to arterial blood sampling and were compared with the results of ABG analysis. The mean (bias) and the standard deviation (precision) of the differences were calculated from the data for each parameter. RESULTS Ninety-four blood samples were obtained. The correlation between CIABG and ABG measurements was strong for each parameter: r = 0.83 (PCO(2)), 0.89 (PO(2)), 0.74 (pH). The bias +/- precision between the two methods was 0.4 +/- 3.0 mmHg for PCO(2), -6 +/- 47 mmHg for PO(2), -1.2 +/- 27.4% for PO(2) and -0.01 +/- 0.04 for pH. For PO(2) values <150 mmHg, the bias +/- precision was 1 +/- 28 mmHg. CONCLUSIONS The agreement between CIABG and ABG measurements was better for PCO(2) and pH than for PO(2). The PO(2) value displayed on the CIABG monitor may not always be reliable during thoracic surgical procedures.