We report a case of Castleman's disease in a 48-year-old male who complained of a rapidly enlarging mass on the neck. Thrombocytopenia and the facio-scapulo-humeral type of progressive muscular dystrophy were also present. The mass was extirpated, and histopathological examinations showed the hyaline-vascular type of Castleman's disease. Following resection, thrombocytopenia was ameliorated. Immunological study with flow cytometric analysis was made to clarify the nature of his mass. With reference to recent literature we suggest that the hyaline-vascular type of Castleman's disease may be an atypical hyperplasia of the lymphoid tissue.