From Jan. 1987 to Dec. 1988, a total of 230 patients (135 lung cancer, 95 breast cancer) were treated by irradiation. The frequency of radiation pneumonitis was 5.1%(7/135) in lung cancer and 3.1%(3/95) in breast cancer. The frequency of lung cancer (n=16) complicated with pleunal effusion and treated by whole chest moving-strip irradiation was 31.2%(5/16) as compared with 2.3%(5/214) in those treated by the conventional radiation. The incidence of radiation pneumonitis was related to dose in the target volume, area of radiation and chemotherapy before radiotherapy.