Three instances of radiation myelitis of the thoracic spinal cord were found among 350 patients treated for carcinoma of the lung, esophagus, and trachea, or for thymic tumors, 40 of whom survived more than 18 months. These 3 patients and the 37 additional patients surviving more than 18 months after irradiation were studied in detail. The time-dose relationships were reviewed and analyzed as a function of number of fractions and of over-all treatment time. The results suggest that the number of fractions is the most important factor in the determination of late radiation damage to the spinal cord. We conclude that radiation myelitis can probably be prevented by avoiding an excessive total dose to the spinal cord and by planning treatment that does not deliver more than 200 rads per fraction.