ICD-9-CM diagnoses for hospitalizations occurring during the first 6 post-transplant months were summarized into Clinical Classifications System (CCS) categories. Of the 28,900 patients examined, 54% had at least one hospitalization. There were 2.39 hospitalizations per patient-year at risk. The total Medicare inpatient costs were $339 million and mean length of stay was 8.3 days. The most common and costly CCS diagnosis was complications of a kidney transplant followed by infections, circulatory system disease, gastrointestinal disease and endocrine complications. The CCS is a useful tool to summarize the complexity of claims information in the USRDS and could prove useful in further claims research.