Clinical signs of Graves' ophthalmopathy were correlated with extraocular muscle volumes in 50 patients. Significant correlations were obtained for horizontal, vertical, and total extraocular muscle limitation, as well as periorbital swelling. Proptosis correlated to a lesser extent. Optic nerve involvement was found to be correlated with both total extraocular muscle volume and limitation of ocular motility. From this information, no risk, future risk, and immediate risk categories of disease were defined quantitatively. The results of the study imply that optic nerve involvement is more likely to occur in association with noncompliant, fibrotic muscle than with more supple muscle of the same total volume.