Treatment of the malignant neoplasms which develop in patients with chromosome fragile syndrome, such as Bloom's syndrome (BS), requires extremely careful planning of the chemotherapy regime because of the excessive chemosensitivity of patients with such syndromes. Two siblings with BS developed B cell-type non-Hodgkin's lymphoma in the third decade of their lives. In both cases a 3-bp homozygous deletion of the BLM gene was detected. Since the lymphoma of the older brother was nasopharyngeal in origin, he was administrated radiation therapy as the primary treatment. However, hepatic metastasis was detected and this was the cause of his death. A 9-bp deletion in exon 7 of the p53 gene was detected in the metastatic lymphoma. His younger sister developed a lymphoma of abdominal lymph node in origin. She received a half dose of the drugs used in the acute lymphoblastic leukemia treatment without radiation, and twenty months after the diagnosis of her lymphoma she continues to be in complete remission and free of treatment complications. The p53 gene mutation was not detected in her lymphoma. These results suggest that radiation therapy and the radiation dose for the treatment for lymphoma in patients with BS should be carefully considered.