Case-mix measurement is a basic requirement of clinical and resource management systems within health care organisations, and offers a potentially useful tool for the setting and monitoring of contracts. Ambulatory care has particular problems in the construction of appropriate case-mix measures, and day-case surgery provides an opportunity to test two existing measures, one inpatient (Diagnostic Related Groups) and one ambulatory (Ambulatory Visit Groups). These grouping systems were applies to the same data to compare the case-mix patterns that they produce. The findings show that the ambulatory visit group appear to have advantages over the diagnostic group with respect to their underlying assumptions and labelling of the groups; in particular, they assign greater weight to procedures. However, diagnostic groups are more developed, easier to use, more familiar and allow direct comparisons with inpatient care. Nevertheless, a proper evaluation of these issues requires further data collection and analysis, together with a fundamental examination of the uses of ambulatory case-mix.