Hypophosphatasia was diagnosed in two boys aged four months and two years. This is a rare hereditary bone disease characterized by deficient activity of enzyme alkaline phosphatase. Increased levels of substrates of this enzyme are found: phosphoethanolamine in urine and pyridoxal phosphate in serum. Patients show defective bone mineralization, which results in severe deformities of limbs, thorax and skull and dental abnormalities (loss of teeth and caries). The disease is classified in four age-related forms: perinatal, infantile, childhood and adult hypophosphatasia. The perinatal form is usually lethal. There is no curative therapy. Recognition of the disease is of importance for genetic counselling.