The causes of raised serum alkaline phosphatase were determined prospectively in a consecutive series of 2884 acute medical admissions. Diagnoses were established by pre-defined clinico-pathological criteria. Two hundred and four patients had hyperphosphatasaemia. Liver and bone disease accounted for only 98 (48%) of the established diagnoses. The diagnosis was not established in 31 (15%). Transient hyperphosphatasaemia occurred in 66 (32.4%) cases. In 25 of these patients (12% of the total) an acute infection was the sole illness identified. In this infected group, the isoenzyme type was predominantly hepatic, although jaundice occurred in only 3 patients. Discriminant analysis revealed that this group was characterized by haematological and biochemical features of an acute phase response. Transient hyperphosphatasaemia is commonly found in acutely ill medical inpatients and is frequently associated with acute extrahepatic infection.