Incontinentia pigmenti (IP) is a rare hereditary neurocutaneous syndrome characterized by typical linear hyperpigmentationed skin lesions, often associated with central nervous system (CNS) involvement, dysplasia in dental and skeletal system, and ocular abnormalities. Thirty to fifty percent of the patients suffer CNS complications such as mental retardation, seizures, spastic paralysis, microcephaly, and cerebellar ataxia. We experienced a case of incontinentia pigmenti in three-month-old female patient who had characteristic linear hyperpigmented skin lesion on both her thighs and partial seizure with secondary generalization. She had family history of typical skin lesions on her maternal relatives. She showed abnormal findings on EEG as well as multiple necrotic lesions on brain MRI. Confirm diagnosis of incontinentia pigmenti was made by skin biopsy. Key Words : incontinentia pigmenti, hyperpigmented skin lesion, destructive encephalopathy Becker muscular dystrophy is a X-linked recessive disease with the affected gene at locus Xp21, characterized by progressive muscular weakness. Without the definite family history, it has been known that the diagnosis of this disease is almost impossible on clinical grounds alone. We reviewed the muscle pathology of two casses of genetically confirmed Becker muscular dystrophy to know the diagnositc significances of this study. The first case, a 20 year old man, is the classical one with definite family history of X-linked recessive heredity. The muscle pathology of the biceps showed dystrophic muscular changes, including increased internal nuclei, marked variation of fiber sizes and mild endomysial fibrosis. The dystrophin stain of the muscle was also confirmative for the diagnosis. The second case was a 32 year old man who has been biopsied his left vastus lateralis 5 years before this genetic diagnosis. This case is a sporadic one without the family history. The diagnosis at the time of muscle biopsy was limb-girdle muscular dystorphy or inclusion body myositis because of the typical rimmed vacuoles and marked variation of fiber sizes. The dystophin stain was not available at that time. Our conclusion is that the molecular genetic study and/or dystrophin protein test of muscle biopsy should be done in every clinically suspected patient, including limb-girdle muscular dystorphy, inclusion body myositis or rimmed vacuolar myopathies.