We have studied the clinical significance of cy to toxic antibodies against human eye muscle cells in patients with thyroid-associated ophthalmopathy (TAO). Eye muscle reactive antibodies were measured in an antibody-dependent cell-mediated cytotoxity (ADCC) assay.A positive test was defined as % specific Iysis greater than the upper limit of normal, taken as the mean plus two standard deviations for normal subjects tested concurrently. As parameters of the severity of the ophthalmopathy we measured the degree of proptosis (mm), level of intraocular pressure (IOP) (mmHg) and American Thyroid Association classes (0–6). ADCC tests were positive in 21 out of 42 patients with TAO and in 8 out of 14 patients with Graves' disease without evident eye disease but in none of 12 normal subjects tested. In patients with TAO mean (±SE) IOP was significantly greater than that in patients with Graves' disease without apparent eye involvement for the primary position and for all gaze positions. There were significant positive correlations between levels of eye muscle reactive cytotoxic antibodies and the severity of the eye disease quantitated as American Thyroid Association classes 0–6, the IOP in the primary position and on downgaze, but not with the degree of proptosis. These results suggest that cytotoxic antibodies, as detected in ADCC, may play a role in the eye muscle damage of TAO and that their measurement may provide a useful clinical test.