An 18-month-old female showed a markedly decreased activity of the lysosomal enzyme β-D-glucuronidase in cultured fibroblasts, leucocytes, and serum. In obligate heterozygotes, the serum and leucocyte β-D-glucuronidase activity was decreased to approximately 50% of the control values. Clinically, the patient had a mild facial dysmorphism reminiscent of Hurler disease, moderate mental retardation, and hepatosplenomegaly. The corneae were clear. Peripheral granulocytes contained abundant, coarse Alder-Reilly granulations. The urinary excretion of acid mucopolysaccharides was increased. The disorder has been classified as mucopolysaccharidosis, type VII. It is assumed that this new mucopolysaccharidosis is caused by the defective action of β-glucuronidase which results in the faulty degradation and intralysosomal storage of acid mucopolysaccharides.