Mucolipidosis III (pseudo-Hurler polydystrophy) is an autosomal recessively inherited Hurler-like disorder without mucopolysacchariduria. Previous reports have noted a constellation of laboratory features similar to that described for mucolipidosis II (I-cell disease). Studies were carried out on a series of 15 patients. Twelve were found to have changes in serum and cultured fibroblasts which consisted of marked elevations of several acid hydrolases in serum with low levels of the same enzymes in cultured cells, a marked increase in dense cytoplasmic inclusions and abnormal radioactive sulfate kinetics. The clinical features of these 12 patients comprise a phenotypic entity. Despite clinical similarity, the 3 remaining patients were not felt to represent mucolipidosis III. The basic defect in mucolipidosis III remains unknown, but is suggested that the defect is similar to that of mucolipidosis II, from which it must be distinguished clinically.