Two patients with post-partum transient thyrotoxicosis associated with painless thyroiditis and low radioactive iodine uptake were described. The surreptitious use of thyroid hormones or iodine was excluded. Although the clinical course was compatible with that of subacute thyroiditis, passing through the hyperthyroid, euthyroid, hypothyroid and recovery phase, the patients showed a sustained elevation of serum anti-thyroglobulin antibody titer during the entire phase of the disease. Moreover, the histological findings obtained by the thyroid biopsy performed in a case were characteristic of chronic lymphocytic thyroiditis. Furthermore, it was of interest that the disease recurred following every delivery in 2 cases, suggesting the possible role of immunological changes induced by pregnancy and delivery in the etiology of this disease.