Cycloleucine (1-aminocyclopentanecarboxylic acid) administration in cancer patients resulted in a reversible marked aminoaciduria consisting predominantly of cystine, ornithine, lysine, and arginine, with only minor elevations of other amino acids. Plasma levels of amino acids were essentially unchanged except for slight decreases in ornithine and lysine. This aminoaciduria is most likely due to a blockage by the drug of the renal tubular reabsorption of cystine, ornithine, lysine, and arginine, resulting in an aminoaciduria of the type seen in cystinuria.