The aim of this study was to evaluate the incidence and risk factors related to secondary thyroid carcinoma (STC) in patients who have undergone allogeneic BMT during childhood. Data related to the primary hematological disorder and BMT procedure were obtained from the records of 113 patients (42 F; 71 M) who underwent BMT before the age of 18 (median 10.0 years; range 1.7-18.0) and survived more than 3 years after transplant with a median follow-up of 10.1 years (range 3.0-19.0). Sixteen received cranial radiation (CRT) during first-line treatment. Pre-transplant conditioning included TBI in 85 patients, TAI in two, while 26 children did not receive irradiation. The standardized incidence ratio of STC after BMT was significantly higher (P < 0.001) than that of the general population. STC was found in eight patients, 3.1 to 15.7 years after transplant. All received TBI and three also CRT. The Cox's regression analysis, although not statistically significant due to the small study population, showed an increased risk in those who had received a cumulative radiation dose higher than 10 Gy and in those who developed chronic GVHD. Careful follow-up of thyroid status including annual ultrasound examination is recommended for early detection of tumor.