According to the DRG classification system, 3% (201/6,954) of the patients treated in a 150-bed surgical department in 1991 were defined as outliers, i.e. their length of stay exceeded the upper limit of a "normal" stay as defined by the DRG system. These patients consumed 20% of total in-hospital days. In 16% of the outliers, hospitalization was extended because of advanced cancer. In 14% of the outliers the stay in hospital was prolonged because an out-patient social service could not be established or the patient could not be transferred to a nursing home. In 35% of the outliers, the disease itself was responsible for the extended stay. Complications following treatment were the cause of prolonged stay in hospital in 35% of the outliers.