Thirty-one patients with chronic lymphocytic leukemia were treated with mediastinal radiation. In none of the patients was complete remission achieved; either partial remission or clinical improvement was achieved in 52 per cent, but the duration of response was short. The response rate was 77 per cent for the patients receiving a total radiation dose greater than 3,000 rads and 45 per cent for those receiving less than 3,000 rads. Severe life-threatening toxicity was noted in 11 patients and seven of these patients died; two patients died with progressive disease. Severe toxicity was manifested by one or more of the following: bone marrow aplasia, pancytopenia, gram-negative sepsis, generalized herpes zoster and severe esophagitis. Neither the total dose of radiation nor the dose per week correlated withe the severity of reaction or death.