Mucolipidosis III (ML-III), or pseudo-Hurler polydystrophy, is an autosomal recessive Hurler-like disorder without mucopolysacchariduria. The diagnosis is challenging for rheumatologists since the musculoskeletal presentation is similar to some rheumatic diseases. We report a case of ML-III in a 16-year-old Taiwanese boy. The characteristic findings of sonography and magnetic resonance imaging (MRI) of claw hand deformity are described. A 16-year-old boy was referred to our rheumatologic clinic because of progressive claw hand deformity, multiple joint stiffness and tightness of the skin over the fingers at the age of 6 years. Sonography and MRI examination disclosed tendon sheath thickening over extensor tendons of both wrists and fingers without features of active inflammation over tendons or joints nor thickening of skin. Urinary glycosaminoglycans were normal. The diagnosis of ML-III was confirmed by the presence of elevated activities of beta-glucuronidase (2141.99 nmol/mg protein/hour), arylsulfatase A (1237.7 nmol/mg protein/hour) and alpha-fucosidase (52.95 nmol/mg protein/hour) in his plasma and decreased activity of these lysosomal enzymes in cultured skin fibroblasts. Sonography and MRI screening for claw hand deformity may offer important clues enabling early diagnosis of ML-III.