The real cost of medical consumption was compared with the proportion of medication consumption of (the) GHM n(o) 681 (homogeneous group of patients, chemotherapy for cancer in day care) in the French case mix system (PMSI). For those patients in our thoracic oncology unit (Sainte-Marguerite Hospital, Marseille, France), the real medication cost was calculated from prices paid by the hospital, then compared to the expected expenditures for the medication consumption of the GHM 681, i.e. 678 French francs (24.1% of the 225 ISA points (synthetic activity index)). Over a period of 2 months in 1998, 87 patients (mean age 63 +/- 11) had 194 chemotherapy sessions in day care, with multi-drug therapy in 38 cases. Vinorelbine or gemcitabine represented 81% of the single drug chemotherapy. In 84% of the single drug and 76% of the multi-drug chemotherapy, the real cost of medication consumption was above the allocated budget. The mean cost for single drug chemotherapy was 1722 FF and 2920 FF for multi-drug chemotherapy. The budget allocated by the PMSI shows a deficit in the most cases. To avoid a restriction in the use of some drugs, it appears that the French system of budget evaluation needs to be improved.