Observation of 39 patients who had undergone surgery for hyperthyroidism with or without endocrine ophthalmopathy between 1965 and 1972 indicate that this type of therapy involves no higher risk than do medication of radiological measures as far as the course of ophthalmic changes is concerned. Examination of the basal and the TRH-stimulated TSH concentrations i.s. argues against any primarily pituitary cause for progressive ophthalmopathy after surgery. In addition, it elucidates the almost inevitable postoperative development of preclinical hypothyroidism while the clinical condition is still euthyroid, and indicates the necessity of constant postoperative hormone substitution.