One case of a 13-months-old-female infant with transient idiopathic hyperphosphatasemia is described. This syndrome is characterized by: 1) increased serum alkaline phosphatase activity not associate with an organic disease; 2) normalization of the enzyme activity within 12 weeks. Familial hyperphosphatasemia, a permanens disease, is also excluded because of ALP normal values in both parents. Rickets, hepatic and biliary diseases are excluded by clinical, radiologic and laboratory data.