Post-partum thyroiditis' prevalence has not been previously evaluated in France. OBJECTIVE To study prospectively a cohort of delivered women to evaluate the prevalence of antimicrosomal antibodies and dysthyroidism during the post partum period. DESIGN, PATIENTS AND MEASURES: 400 women were observed 4 to 12 weeks after delivery and evaluated for the presence of a goiter and a measure of antimicrosomal antibodies (AMA) and us TSH. A subgroup of these selected women was then prospectively followed. HLA typing was also performed. RESULTS 4 to 12 weeks after delivery, a goiter was found in 40 women. 19 women had positive AMA levels. No abnormality in thyroid function was detected during this period. In these selected women, 8 cases of dysthyroidism, mainly hypothyroidism, were diagnosed later on, 3 to 9 months after delivery. One of them became AMA positive during the follow-up, so the AMA prevalence was 5%. HLA DR4 and the hyplo-type HLA A3B7 were more frequent in this selected population. CONCLUSIONS Post-partum thyroiditis is a rare and benign disease in France. The transient dysthyroidism occurs later than 3 months after delivery, mainly in women with positive AMA (5% of the parturients). HLA DR4 is more often encountered in this thyroid pathology, as already observed in other Caucasian populations.