Transient hyperphosphatasemia of infancy. Striking transient increases in serum alkaline phosphatase in nine new cases are reported. The etiology is still unknown. The patients were 6.5 months to 30 months of age and none showed any clinical evidence of hepatic or skeletal disease. Values became normal within 12 weeks in all patients. It is important to know this condition to avoid unnecessary and expensive diagnostic procedures. Though hyperphosphatasemia does not seem to give any problems we recommend a brief diagnostic protocol in order to recognize transient hyperphosphatasemia of infancy.