The contribution of physiological values to the APACHE II score was determined by retrospective analysis of 11348 patients undergoing intensive care. Eleven physiological variables contributed a mean of 8.9 points, 54% of the total APACHE II score. The mortality ratio (observed hospital mortality/hospital mortality predicted by APACHE II) was 1.13. We altered the APACHE II scores and post-intensive care hospital mortality in order to examine the effect on the mortality ratio of these changes. Increasing scores by two or four points decreased mortality ratios to 1.00 and 0.89 respectively: decreasing scores by two or four points to a minimum of zero increased mortality ratios to 1.27 and 1.44 respectively. A 25% increase or decrease in post-intensive care hospital mortality changed mortality ratios to 1.21 and 1.05 respectively. Physiological values vary with the timing of collection and accuracy of recording. Small consistent differences in scores cause potentially important changes in the mortality ratio. Unless data collection and the effect of management before and after intensive care are standardised, using mortality ratios to compare intensive care units is likely to be inaccurate and misleading.