We present a case of benign transient hyperphosphatasemia in a 4-month-old infant with acute bronchiolitis and pneumonia. During hospitalization the infant had an increased catalytic activity of alkaline phosphatase (ALP): day 2, 5074 U/L; day 3, 5622-U/L; and day 8, 3129 U/L. The x-ray, leukocytosis, and C-reactive protein findings pointed to bacterial etiology of the respiratory disorder. Electrophoretic separation revealed an atypical isoenzyme profile: fast anodal, near-cathodal and bone fractions. ALP levels normalized within 54 days, and control electrophoresis indicated normal liver, placental/placental-like, intestinal and bone isoenzymes. The appearance of atypical fast anodal and near-cathodal fractions of ALP in this infant during the course of acute lower respiratory tract infection and rapid return to the reference intervals pointed to benign transient hyperphosphatasemia.