We evaluated the indices and efficacy of differentiation induction by all-trans retinoic acid (ATRA) in the treatment of differentiated thyroid cancer. METHODS: Fifteen cases of dedifferentiated thyroid cancer, including 5 cases of follicular carcinoma, 8 cases of papillary carcinoma and 2 cases of mixed follicular-papillary carcinoma. The iodine uptake of metastases, the size of metastases and serum thyroglobulin (Tg) level were measured before and after ATRA-induced differentiation therapy. RESULTS: In 15 patients treated with ATRA-induced differentiation therapy, I-131 uptake of lesions was increased in 7 cases; metastases were reduced in 7 cases; among the 12 patients whose Tg was measured, 4 cases (33%) had both decreased fly, increased iodine uptake of lesions and reduced or no change in lesions. CONCLUSION: ATRA has some efficacy in dedifferentiated thyroid cancer and warrants further study.