Introduction Transient hyperphosphatemia (TH) is a benign condition, characterised by transient increase of the activity of serum alkaline phosphatase (ALKP). This condition is usually found in children under 5 years of age and elevation of ALKP activity does not last more than 4 months.

Objectives To clarify if there is evidence of bone and liver disease when activity of bone isoenzymes of ALPK is markedly elevated.

Material and methods Herein we present three infants, aged 5,12 and 18 months. All of them had febrile viral respiratory infection, which required hospitalisation. All of them had increased ALKP 637, >1000, >1000 U/l. Liver enzymes were normal as well as the values of serum calcium and phosphorus. All children have had regular antirachitic prophylaxis. The serum ALKP normalised within 2–3 months. Increased values of ALKP were initially considered as a sign of rickets by the paediatrician who treated infants for respiratory infection and vitamin D therapy was recommended, but was not implemented after reconsideration.

Conclusion Infants with TH can be clearly identified from those having rickets by considering the age of the patient, regular vitamin D prophylaxis, history of viral infection and by excluding other causes of elevated ALKP, particularly liver or bone disease.