Radioiodine (I-131) therapy is of proven efficacy for differentiated thyroid carcinoma. However, its efficacy relies on specific uptake mechanisms, which may be lost during the evolution of the disease. Attempts to increase the iodine uptake of such tumors have been made using retinoic acid because it exerts redifferentiating effects on thyrocytes. This study aims to assess the capability of the retinoic acid (RA) treatment to reinforce iodine 131-irradiation efficacy for metastatic and progressive multi-irradiated thyroid cancer. In this clinical prospective study, 11 patients (mean age +/- 1 SD = 61 +/- 12 years, sex ratio M/F = 5/6) with a progressive disease despite iterative surgery and iodine irradiations were treated with 13-cis-retinoic acid (1.5 mg/kg day) over 8 weeks prior to I-131 irradiation. The redifferentiating effect of RA was evaluated by serum thyroglobulin (Tg) monitoring during RA treatment and qualitative analysis of iodine uptake on the post-therapeutic whole body scan. The clinical usefulness of RA treatment was assessed by clinical follow-up, Tg monitoring, and tumor size. No serious event that could possibly be related to the treatment was reported. The mean follow up time was 24.2 +/- 12 months (range 3-46 months). Iodine uptake was only slightly improved in two patients. Nevertheless, the clinical benefits of RA seem to be very poor. Five patients died of a metastatic disease. Five others presented new clinical evidences of a progressive disease. In conclusion, this prospective study demonstrates the absence of efficacy of I-131 irradiation combined with RA for the treatment of patients with aggressive, rapidly growing metastatic thyroid cancer. Thus, patients with highly aggressive disease, rapidly growing in a short period from 2 to 6 months, should not be considered for RA therapy.