The phosphoethanolamine (PEA) concentration in morning urine was assessed by liquid chromatography in 866 patients examined because of suspected impaired aminoacid metabolism. In 763 patients the blood was also examined. A serum concentration of PEA above 10 mumol/l was recorded in 2.1%. The concentration of PEA in urine above 10 mmol/mol creatinine was recorded in 42%. The authors revealed a significant relationship between urinary PEA excretion and the patient's age. The urinary PEA concentrations are higher during the first weeks and months of life, in older children and during adolescence its excretion declines and this trend was recorded also in the author's group of patients. In a group of 111 infants with impaired function of the CNS, in 66 infants with systemic skeletal affections and in 73 infants with hepatopathies a significantly higher mean urinary PEA concentration was found that in the control group of healthy infants. In children aged 3-14 years and in the group of older children the mean PEA concentration was elevated only in patients with systemic skeletal affections, even after elimination of patients with hypophosphatasia. Long-term or intermittently increased PEA excretion is a manifestation of specific metabolic disease only in hypophosphatasia. In other diseases it can be interpreted as a secondary finding conditioned by an impaired phospholipid metabolism at the level of cellular membranes in the CNS, liver or skeleton.