Many clinicians have the impression that treatment of thyroid dysfunction ameliorates ophthalmopathy in Graves' disease. The aim of our study was to relate thyroid function to the severity of Graves' ophthalmopathy. We studied 90 patients with Graves' ophthalmopathy and Graves' hyperthyroidism in whom severity of Graves' ophthalmopathy and thyroid function (regardless of antithyroid treatment) were assessed when referred to our institution. Patients were assigned to four groups (A through D) with increasingly severe Graves' ophthalmopathy using Total Eye Score based on the NOSPECS classification. The percentage of dysthyroid patients in each group was determined: group A had 23% dysthyroid patient (5); group B, 32% (9); group C, 61% (14); and group D, 47% (8). More dysthyroid patients were in the groups with severe Graves' ophthalmopathy. We also compared the severity of Graves' ophthalmopathy between euthyroid (n = 54) and dysthyroid (n = 36) patients: euthyroid patients had less proptosis (19.9 +/- 3.5 vs 20.8 +/- 3.4 mm), better visual acuity (0.93 +/- 0.17 vs 0.88 +/- 0.18), and lower Total Eye Score (8.6 +/- 6.6 vs 10.6 +/- 6.6). We conclude that dysthyroidism is associated with more severe Graves' ophthalmopathy. Our findings suggest that meticulous control of thyroid function also during antithyroid treatment is important in the management of Graves' ophthalmopathy.