A 2-year-old girl with reducing body myopathy was reported. She had no family history of neuromuscular disease. She developed normally with no delay in milestones during infancy. She had no muscle weakness or hypotonia up to 2 years of age when she received mumps vaccination. Three days after the injection, she was first noticed to have limb muscle weakness. The muscle weakness progressed rapidly with increasing difficulty in gait and raising the upper arms, particularly the left. Four months later, she had difficulty in keeping her head up and could no longer climb the stairs. On physical examination, she had proximal dominant generalized muscle weakness, with a preferential neck muscle involvement. She walked waddlingly and stood up with Gowers' maneuver. Facial and ocular muscles were intact. No dysarthria, dysphagia or respiratory difficulty was noted. EMG showed myopathic pattern. Serum creatine kinase level was moderately elevated to 739 IU/l. In the biopsied left biceps muscle, there was marked variation in fiber size, but no apparent necrotic or regenerating fibers. The most striking feature was the presence of numerous eosinophilic inclusions which reduced nitroblue tetrazolium (NBT) and were, therefore, stained dark with menadione-linked alpha-glycerophosphate dehydrogenase even without the substrate of menadione, showing the histochemical characteristics of" reducing" body. The bodies were predominantly seen in fibers with disorganized intermyofibrillar networks and with high acid phosphatase activity. On electron microscopy, the reducing bodies consisted of fine granular material with the similar electron density to the chromatin granules and were located mostly around the degenerated nuclei, suggesting the nuclear degeneration playing a role in forming the reducing bodies.(ABSTRACT TRUNCATED AT 250 WORDS)