We have examined the pattern of four radiation-related neoplasms in a radiation-exposed cohort of 2795 patients. They had received conventional radiation treatments for benign conditions in the head and neck area when they were children. At the end of follow-up, 350 thyroid cancers, 108 benign and malignant salivary tumors, 87 neural tumors, and 70 cases of hyperparathyroidism had occurred. In 492 individuals (17.6% of the cohort), there were single tumors, and in 60 individuals (2.1%), there were multiple tumors. Because this was an apparent excess of multiple tumors, we tested the hypothesis that the excess arose, at least in part, from variation in radiation susceptibility in the cohort. To analyze this, we developed a method to account for differences in length of follow-up and known risk factors, such as age at radiation exposure, radiation dose, and gender. This analysis showed that the excess concordance of neoplasms could be explained by known risk factors, thereby suggesting that susceptibility factors did not play a role in the development of multiple tumors. Of the 350 thyroid cancers, 50 occurred in subjects with other radiation-related neoplasms. Therefore, we tested an additional hypothesis, that the presence of these other neoplasms was related to the clinical behavior of the thyroid cancer. Using thyroid cancer recurrence as the end point, we did not observe a relationship with the presence of other neoplasms. In summary, we demonstrated an excess of concordance of radiation-related neoplasms that could be explained by known risk factors, and we found that thyroid cancer behavior was not related to the occurrence of multiple tumors.