Aim. To determine the relative risk of developing other malignant tumors in patients with thyroid cancer and and the risk of developing thyroid cancer in patients with other malignancies. Methods. A retrospective analysis of 116 patients with multiple neoplasia including thyroid gland involvement was conducted for the period from 1973 to 2010. In order to estimate the relative risk of development of multiple neoplasias including thyroid gland lesions used was the following formula: relative risk = [a / (a + b)] / [c / (c + d)], where a is the number of patients with thyroid cancer with a second malignancy; b is the number of patients with thyroid cancer without a second malignancy; c is the number of patients in the population affected by the same malignant disease, as patients in group a; d is the number of people in the population without any cancer-related pathology. Results. In patients with carcinomas of the thyroid gland the relative risk is higher than in the general population for developing metachronous lymphoma (41.8 for men, 31.7 for women), renal cell carcinoma (55.6 for men, 18.5 for women), prostate cancer (35.7), lung and bronchus cancer (18.8 for women), melanoma (17.1 for women), colon cancer (16.7 for women), cervical cancer (15.8), uterine cancer (11.8), breast cancer (11.5 for women), skin cancer (9.5 in women) and the simultaneous development of renal cell carcinoma (33.8 for men and 46.3 for women), prostate cancer (24.4), melanoma (20.6 for women), cancer of the esophagus (19.4 for men, 17.8 for women), colon cancer (19.0 for men), lymphomas (12.8 for men), cervical cancer (11.3), breast cancer (11.0 for women), skin cancer (8.5 for women). The relative risk of developing metachronous cancer of the thyroid gland is higher than that in the population in patients with melanoma (108.0 in men, 50.4 for women), with malignant neoplasms of the lymphoid tissue (40.2 for men, 40.8 for women), uterine cancer (11.8), skin cancer (8.7 in women), breast cancer (8.0 for women). Conclusion. During preventive medical examinations of patients with thyroid cancer the relative risk of developing subsequent cancers must be taken into account for early diagnosis of multiple neoplasias.