Background Second malignant neoplasms (SMNs) have become a primary concern in evaluating long‐term effects of treatment in pediatric oncology. Thyroid carcinoma has proven to be a common SMN. Methods. In a multicenter study involving 58 hospitals in Germany, Austria and Switzerland, 18 of 239 (7.5%) SMNs documented following first malignant neoplasm (FMN) in childhood were thyroid carcinoma. Results. The age at diagnosis of FMN ranged from 1 to 15 years. Eleven patients were female. Six children had survived Hodgkin disease, seven acute leukemia, two Ewing sarcoma and three various other tumors. Fifteen of the 18 patients had been treated with radiotherapy to the head and neck region. The time interval between treatment and diagnosis of thyroid carcinoma ranged from 4 to 19 years (median 8 years). The carcinoma was papillary in 17 cases and follicular in one. Eleven patients had metastases in the regional lymph nodes at presentation. Discussion. Radiotherapy appears to be an important risk factor in secondary thyroid carcinoma, but it does not explain all cases. The current data are remarkable for the large proportion of patients who received only prophylactic cranial irradiation for ALL and for the three patients who received no irradiation to the head and neck region. Genetic determinants and chemotherapy must also be considered. Conclusions. Regular thyroid examination should be included in the long‐term follow‐up of survivors of childhood malignancy. Med. Pediatr. Oncol. 31:91–95, 1998. © 1998 Wiley‐Liss, Inc.