OBJECTIVE: All chemotherapy regimens are associated with some degree of adverse events. The more severe adverse events require hospitalization and may be associated with high costs. One adverse event that may be serious is infection and in particular infection because of neutropenia. The objective of this study was to retrospectively assess the hospitalization costs of infections and neutropenia in cancer patients. METHODS: Individual patient data on costs, diagnoses, and length of stay were collected from the largest cost per patient inpatient database in Sweden. The time period was January 1999 to January 2000. The hospitals included in the database all have a detailed resource tracking and cost assignment system for determining the individual cost per stay. All non-surgical patients who had the combination of a cancer ICD-10 (C000 to C997) and an infection diagnosis recorded in the database were selected. Patients who also had a neutropenia (D709) diagnosis recorded were selected and studied as a subsample of the whole sample. RESULTS: There were 2378 patients who had a cancer and an infection diagnosis. Their mean cost was (SEK) 69,700 and the mean length of stay was 12.3 days. The average age was 62 years and there were 59% women. Patients with a principal cancer diagnosis had greater costs than patients with a secondary cancer diagnosis, 85,500 versus 50,600. Out of the 2378 patients there were 52 who had both neutropenia and an infection. Their mean age was 55 years. There were slightly more women than men, 54%. The mean cost was (SEK) 77,900 and the mean length of stay was 12.9 days. CONCLUSIONS: The hospitalization costs of infections and neutropenia in cancer patients are significant. When assessing the costs of chemotherapy treatments, not only pharmaceutical costs, but also costs of adverse events should be included.