A previous survey of patients who survived more than 6 months after radiotherapy for carcinoma of the bronchus using a 6 fraction regimen revealed a considerable incidence of radiation myelitis. In a further survey, in which the data bank has been increased from a total of 303 to 754 cases, analyses have confirmed that radiation myelitis occurs once a threshold dose of 33.5 Gy to the spinal cord has been reached. The incidence was positively related to the hemoglobin concentration, but not to the blood pressure at the time of radiotherapy. In the same group of patients survival was positively related to radiation dose, the hemoglobin concentration, and the systolic blood pressure. In other patients who were treated with 6 fractions, but who received a lower minimum tumor dose, either because this was planned or as a result of cord shielding, no relationship was shown between survival and radiation dose, hemoglobin concentration and systolic or pulse pressure. Radiosensitivity is dependent upon the oxygen concentration which, in normal tissues, is related to the hemoglobin concentration and in tumor to both the hemoglobin and the systolic blood pressure. The achievement of a threshold radiation dose appears essential before these prognostic factors become relevant.