BACKGROUND: In differentiated thyroid carcinomas (DTC), it has been reported that pregnancy may accelerate the course of the disease. But recent evidences suggested that the prognosis of DTC during pregnancy was similar to that of DTC in non-pregnant women of the same age. Also the optimal timing for the treatment is still controversial. We evaluated the clinical features of DTC in pregnant women. METHOD: We reviewed the histories of patients in whom the DTC was diagnosed before or during the pregnancy between 1994 and 1999. DTC were diagnosed by fine needle aspiration and the patients were treated by thyroid surgery. RESULTS: Six women who had a mean age of 30 years (27-34 years) were identified. The mean follow-up duration was 41 months (13-70 months). All patients had noticed a lump in their necks. In three patients, the nodules increased in size during pregnancy. A fine needle aspiration revealed a suspected malignancy in five patients and a postoperative biopsy confirmed the malignancy in one patient who had a preoperative cytologic diagnosis of nodular hyperplasia. All tumors were well differentiated and ranged in size from 1 to 6.5 cm. Radioactive iodine ablation and thyroid hormone suppression treatment were administered in five patients except in one case of papillary microcarcinoma. One patient had residual tumors in the right cervical lymph nodes and both lungs. She underwent repeated surgery and radioactive iodine therapy. CONCLUSION: This reports suggest that the DTC which is associated with pregnancy may have a similar prognosis to that of non-pregnant women and that the treatment of DTC in pregnant women may be safely delayed until after delivery in most patients. The treatment should not be delayed for more than a year.