Spasticity and severe psychomotor retardation in a 2-year-old girl, apparently in expression of chronic progressive encephalopathy, was found in association with prolinuria, hydroxyprolinuria and hyperglycinuria. The pattern and levels of these amino acids in plasma proved to be normal, however, as was their intestinal absorption as well as their concentration in cerebrospinal fluid. The proband appears to be homozygous for iminoglycinuria with an apparent inborn defect of the renal tubular transport system specific to these amino acids. An isolated hyperglycinuria, however, was traced in several obligate heterozygotes in this family. The possibility of a more direct causative link between this anomaly and her clinical manifestations is discussed. Where instances of iminoglycinuria occur in association with cerebral pathology of this type, an analogous impairment of a correspondingly specific cerebral amino acid transport system is postulated.