Purpose：The chronic use of immunosuppressive therapy in transplant recipients can increase the long-term risk of carcinoma. The aim of this study was to determine the incidence, biological behaviors, and treatment outcomes in PTC(papillary thyroid carcinoma) in renal allograft recipients. Material and Methods：The present study examined the incidence and biological behavior of PTCs in RA recipients. A total of 1,739 RA patients treated between January 1986 and December 1999 were followed-up for a median 137(84-238) months. During the follow-up period, 129(7.4%) recipients were identified as having posttransplant malignancies. Of those, 12(0.7%) had PTCs, and these comprised six male and six female patients with a median age of 41(23-57) years. Results：Nine cases(incidentalomas) were diagnosed based on ultrasonography(US) screening. Eight of those nine were TNM stage I, and two of the three clinical carcinomas were TNM stage IVa. During a median follow-up of 94(18-159) months, two(16.7%) PTC patients developed loco-regional recurrence, but no patients showed distant metastasis. Posttransplant PTC showed no gender bias, and was often associated with aggressive lymphatic metastasis. However, most incidentalomas showed a favorable treatment outcome. Conclusion：In conclusion, routine surveillance of the thyroid gland using US screening is recommended to ensure early detection, treatment and favorable prognosis in RA patients with PTC.