Congenital fiber type disproportion (CFTD) has been described as a form of congenital myopathy characterized by the smallness and marked predominance of type I fibers in a muscle biopsy. Only major morphological characters in CFTD are the abnormality of the size of muscle fibers and the disproportion of the type of muscle fibers. Clinical feature of CFTD is characterized by congenital hypotonia, nonprogressive muscle weakness and delayed motor milestones. The disease is sometimes associated with a myopathic pattern in the electromyography (EMG) and a slightly increased creatine kinase (CK). In this report, we describe a case of the child presented the subtle clinical symptoms of mild proximal weakness of lower extremities, who was diagnosed as CFTD not by the laboratory findings such as EMG and muscle enzyme study of CK initially but with a muscle biopsy finally. (J Korean Acad Rehab Med 2002; 26: 485-488)