99 A temporary elevation of serum alkaline phosphatase (ALP) activity up to 70 times the upper normal value for age without evidence of liver or bone disease is known as "benign transient hyperphosphatasemia of infancy and childhood". The role of infectious agents has been suggested. We report three girls (10-28 months) with diarrhea and rotavirus infection who had severe hyperphosphatasemia (1033, 2589, 2764 U/L). They had normal serum calcium, phosphorus, parathyroid hormone, and 1,25 dihydroxycholecalciferol levels. Their bone radiogram revealed no rickets. Serum transaminases. gamma-glutamyl transferase, lactate dehydrogenase, 5′ nucleotidase, and bilirubin levels were normal. Two of them had a small bowel biopsy that was normal. The ALP isoenzymes were predominantly of skeletal origin in all subjects. With no evidence of bone or liver disease, serial measurement of serum ALP levels revealed a spontaneous normalization (ALP: 149-246 U/L) within 16 to 42 days, and there was no sequelae. This benign transient hyperphosphatasemia may be associated with or induced by diarrhea or rotavirus infection. The mechanism of elevated serum ALP levels remains elusive. An acute infection in the presence of diarrhea may lead either to decreased clearance or to increased production of skeletal ALP isoenzymes secondary to hyperactivated osteoblasts. When the level of hyperphosphatasemia is significantly greater than what is expected from rickets or cholestasis, children younger than 5 years who have rotavirus infection require serial measurement of ALP levels only. However, if ALP remains elevated after 8 weeks of the initial determination, further studies become indicated. Awareness of this phenomenon will curtail expensive and extensive investigations.