Variation in the gross anatomy of the thyroid gland is relatively common. Although thyroid hemiagenesis is felt to be a rare anomaly, its incidence is probably underestimated as the diagnosis is usually incidental. The case of a 41-year-old woman with right thyroid hemiagenesis associated with papillary adenocarcinoma is presented. The diagnosis of hemiagenesis was established by isotope imaging and surgical exploration for a benign nodule. Seven years later she was seen with a recurrent neck mass, and an isotope scan revealed it to be a cold thyroid nodule. As she was diagnosed to have papillary adenocarcinoma, total thyroid lobectomy was performed and at present she remains disease-free.