The accuracy and reliability of an invasive intra-arterial oxygen sensor catheter was evaluated in 20 critically ill surgical intensive care unit (SICU) patients. All patients required continuous arterial blood pressure monitoring, at least 72 hours of ventilator support, and intermittent arterial blood gas sampling for clinical management. The intra-arterial sensor provided continuous PO2 (PsO2) values on a bedside electronic monitor. PsO2 values were sampled every 60 seconds and automatically stored on a bedside personal computer. Arterial blood gas (ABG) PaO2 values were collected and matched by collection time with corresponding PsO2 values. During 1,238 hours of continuous intra-arterial monitoring, 74,280 PsO2 values and 246 ABG PaO2 values were collected. Of the 246 PaO2 results, 175 (71.3%) had a matching PsO2. Regression of matched PsO2 and PaO2 values yielded a correlation coefficient of 0.58 and standard error of the estimate (SEE) of 33.1 (P < 0.0005). Even though matched PsO2 and PaO2 measurements demonstrated a linear relationship, only 34 per cent of the variation in PsO2 could be attributed to changes in PaO2. Technical sensor or instrument problems affected PsO2 monitoring in 17 of 20 patients and 28 of the 33 sensors tested. The authors conclude that continuous intra-arterial monitoring of PsO2 is a novel idea, but technical issues limit its use in acutely ill, conscious SICU patients.