OBJECTIVE: To assess the appropriateness of the current way chemotherapy is classified in the Australian casemix classification system. METHOD: Secondary analysis of patient-level data for all cases of DRG 780 separated from Victorian hospitals with clinical costing systems in the years 1994/95, 1995/96 and 1996/97 using the Chi-squared Automatic Interaction Detector (CHAID) option of Answer Tree Version 2.0. RESULTS: Different cancers have different costs. There is a significant difference (F=316.8, df=4, 11091, p=0.00) between the cost of colon cancer (mean=$289), breast cancer (mean=$481), lymphatic cancer (mean=$605), leukemia (mean=$1,118) and other cancers (mean=$512). The differences are sufficiently large that they meet the statistical criteria for splitting a DRG. CONCLUSION: Application of nationally agreed criteria for creation of Diagnosis Related Groups suggests that the existing DRG 780 should be split into five groups: colon, breast and lymphatic cancers, leukemia and a residual 'other cancers' category. The new groups increase the explanation of variation in costs (about a 10% reduction in variance). IMPLICATIONS: It is no longer valid for casemix development to be based on length of stay as a proxy for resource utilisation, especially for DRGs that are primarily same day.