The clinical characteristics of a 16-year-old white girl with mucolipidosis type III included early growth retardation, severe dysostosis multiplex, restricted joint motion, tight indurated skin, swollen eyelids, late-onset hepatosplenomegaly, umbilical hernia, corneal opacities, and only slightly impaired mental and neurological development. Cultured fibroblasts contained numerous coarse perinuclear retractile inclusions. Biochemical findings indicated the following: (1) normal levels of urinary acid mucopolysaccharides, (2) deficient activities of multiple lysosomal hydrolases in cultured fibroblasts, (3) elevated activity levels of seven serum lysosomal hydrolases, and (4) elevated activity levels of four lysosomal hydrolases in urine.