A literature review and ten new cases of benign transient hyperphosphatasemia of infancy are presented, with special attention paid to isoenzyme studies. Polyacrylamide gel electrophoresis, heat denaturation, and binding of alkaline phosphatase to anti-human alkaline phosphatases showed that the sources of the elevated alkaline phosphatase levels are normal bone and liver and not the small intestine. The data also suggest that the following criteria be present for a diagnosis of transient hyperphosphatasemia: (1) an age of less than 5 years, (2) variable symptoms, (3) no bone or liver disease noted on physical examination or (4) from laboratory investigations, (5) isoenzyme analysis showing elevations in both bone and liver activity, and (6) a return to normal serum alkaline phosphatase activity values within four months.