Although combination therapy for keloid including postoperative radiation therapy (RT) is common, the radiation toxicity of RT in a patient with a history of collagen vascular disease has not been fully recognized. We experienced a case of an acute radiodermatitis in a patient with keloid. This patient had a chest keloid because of the bypass surgery for Takayasu's arteritis. After we performed an excision and postoperative RT, severe radiodermatitis occurred. We speculate that the higher single dose and the use of electron beams may be related to the onset of severe acute radiodermatitis in this case. It should be kept in mind that there is a risk of exacerbation of radiation toxicity in patients with collagen vascular disease.