Thyroid agenesis would appear to be a rare abnormality (50 cases, including 8 personal). In fact, it is a simple anatomical curiosity which may be found in the presence of any type of thyroid condition (simple goitre: 4 cases, thyrotoxicosis: 2 cases, cold nodule: 2 cases, as far as our own patients were concerned). Our 8 patients were all women, hemiagenesis being more commonly discovered in the female sex. In 2 patients the right lobe was absent, and the left in the other 6. In general, the abnormality affects the left lobe three times more commonly than the right. The differential diagnosis essentially lies with autonomic thyroid nodules, which are much more common, and isotope scanning provides the answer.