A comprehensive study was undertaken of two patients who had typical phenylketonuria (PKU) and chronic dermatitis. Hematologic, immunologic, and histologic (light and electron microscopy) studies and quantitative intracellular amino acid measurements were made from the skin and cultured fibroblasts of these patients. Findings were: (1) the complement receptor-bearing lymphocytes (EAC rosettes) were decreased; (2) both light and electron microscopic findings revealed nonspecific dermatitis; (3) fibroblastic studies suggested little or no phenylalanine hydroxylase activity in cells of patients with PKU and controls; (4) the intracellular concentration of phenylalanine was significantly higher in the skin of PKU patients than in the controls; and (5) patch tests using 50% phenylalanine in petrolatum were negative in four untreated adult PKU patients and four controls.