Seven patients with renal failure of varying etiology had abnormalities in urinary excretion and plasma content of the imino acids (proline and hydroxyproline) and of glycine. Urinary excretion of proline and free hydroxyproline was increased, while peptide hydroxyproline and glycine excretion were normal. Plasma levels of the imino acids and glycine were elevated. The metabolic abnormalities appeared in one patient as renal failure developed and disappeared in two patients after successful renal transplantation. Partial parathyroidectomy or calcium infusion in two patients decreased the iminoaciduria. Osteitis fibrosa was present in four of six bone specimens. The findings suggest that the iminoacidopathy is acquired during the course of chronic renal failure, due in part to secondary hyperparathyroidism.