Clinical pathways are commonly developed for homogenous patient groups. We were wondering if the traditional patient classification systems could be used for classifying clinical pathways.To examine the utility of patient classification systems for clinical pathways, a sample of 13 clinical pathways was analyzed, involving a total of 412 patients. Three classification systems were tested: International Classification of Diseases, Ninth Revision (ICD9-CM), Clinical Coding System (CCS) data and All-Patient Redefined Diagnosis Related Groups (APR-DRG).Categorization with ICD9-CM and CCS shows rather wide variation. However, when restricting for the principal codes, CCS classification shows an almost homogeneous relationship with clinical pathways. APR-DRG's are already corrected for secondary procedures and are difficult to assess. Categorization with the Risk Of Mortality (ROM) is more homogeneous than with the Severity Of Illness (SOI).Patient groups in clinical pathways are rather heterogeneous. When restricting for the principal procedures, the strongest relationship seems to exist between clinical pathways and CCS. Further research is needed to refine this relationship.