We report on a patient of Ki-1 lymphoma-associated hypereosinophilic syndrome and eosinophilic polymyositis. A 22-year-old male was admitted to a hospital because of anterior chest pain, when he had a white cell count of 12,000 with 33% of eosinophils. He first noticed muscle weakness in the right shoulder at age 23 years. At the age of 25, he experienced marked muscle weakness in his arms and neck, which brought him to our hospital. Neurological examination on admission revealed moderate muscle atrophy and weakness in the neck flexors and both upper extremities, and marked muscle hypertrophy in the sternocleidomastoideus and trapezius muscles. Laboratory studies showed a white blood cell count of 17,700 with 56% of eosinophils. His serum creatine kinase (CK) level was elevated to 347 units. A muscle biopsy specimen showing active myositis infiltrated by eosinophils confirmed the diagnosis of eosinophilic myositis. Although eosinophilia and serum CK activity decreased markedly with corticosteroid treatment, his muscle weakness was unchanged. At the age of 27 years, lymphadenopathy developed. Immunocytochemical examination on a biopsied neck lymph node indicated Ki-1 lymphoma with positive Ki-1, CD45 and epithelial membrane antibodies. In spite of chemotherapy and local irradiation treatment, he died four months after the emergence of Ki-1 lymphoma. We discussed the relationship between hypereosinophilic syndrome, eosinophilic polymyositis and Ki-1 lymphoma on pathogenetic mechanism.