Summary Three cases of hemiagenesis of the thyroid gland are reported and the literature reviewed. It is emphasized that this rare congenital anomaly is seldom diagnosed except by thyroid scanning or at surgery. If the clinician is aware of it, the diagnosis presents little difficulty and an unnecessary operation may be avoided. All our cases were discovered as a result of an associated thyroid disorder. In 2 cases there was an adenoma and in 1 a papillary adenocarcinoma.