The excretion pattern of urinary acid mucopolysaccharides was determined in 72 patients with different clinical types of mucopolysaccharidoses and mucolipidoses. Using a chromatographic fractionation method, characteristic excretion patterns were found in the six classical types of mucopolysaecharidoses. Patients with Hunter's disease excreted relatively more heparansulfate and less dermatansulfate than patients with Httrler's disease. In Sanfilippo's disease the excretion of heparansulfate only was increased. In Morquio's disease abnormal amounts of substances with characteristics of chondroitin-4(6)-sulfate were found in addition to keratansulfate. In one patient with Ullrich-Scheie's disease and six patients with Maroteaux's disease relatively large amounts of substances with characteristics of dermatansulfate were present. There was very little overlap between the excretion patterns of different types of mucopolysaccharidoses. In the types of mucolipidoses investigated, the urinary excretion of acid mucopolysaccharides was normal. The intrafamilial variability in eight pairs of related children was small. Our data suggest that the excretion pattern of urinary acid mucopolysaccharides, as determined by the Dowex Column Chromatography Method, is a valuable aid in the definition of the mucopolysaccharidoses.