We describe the case of a 30-year-old woman who, five months after giving birth, was referred with a solitary nodule in her anterior neck. Laboratory analysis, ultrasonography, pertechnetate (Tc99m) thyroid scan and cytological examination of fine needle aspiration biopsy performed on the nodule led us to diagnose postpartum thyroiditis (PPT). Twenty-eight months after parturition, overt hyperthyroidism developed, with raised thyroperoxidase and thyroid stimulating hormone receptor antibody titres, diffuse high uptake of Tc99m at thyroid scan, and high vascular flow throughout the gland at Color-Power imaging. The diagnosis of Graves' disease (GD) was established. The differential diagnosis of thyrotoxicosis in the postpartum period, and the possible aetiological relationships between PPT and GD are discussed. To our knowledge, this is the first published report of a PPT presenting as a cold nodule, and evolving to GD.