Calcium and calcium-regulating hormones were investigated in 19 infants with phenylketonuria. Parathyroid hormone, osteocalcin, dehydrocholecalciferol were found elevated while calcitonin reduced in the serum of the infants prior to the diet. Total calcium and phosphorus remained stable in the course of the study. Element diet resulted in reduced serum parathormone levels though surpassing those of healthy subjects. This fact is attributed to primary metabolic disorders responsible for the onset of hyperparathyroidism. The conclusion is made that element diet is unable to change calcium homeostasis and that of calcium-regulating hormones in infants with phenylketonuria.