The isoenzymes of serum alkaline phosphatase (AP) were determined in 17 children and 3 adults who had transiently or persistently raised serum AP levels without clinical, laboratory or radiographic evidence of hepatobiliary or bone disease. Liver and bone AP were both raised in 8 of 11 children having transient hyperphosphatasia whereas a rise in only one of the two enzymes (liver- or bone-AP) was detected in the serum of children and adults having persistent hyperphosphatasia. Isolated hyperphosphatasaemia is an anomaly having no pathological significance which, when found, has importance since expensive and invasive investigation methods that come into question can be avoided.