We investigated an unselected series of 55 patients with treated or untreated hyperthyroid Graves' disease, assessing their clinical and laboratory status and ophthalmological findings, including the difference in intraocular pressure (dIOP) between upgaze and straight gaze using applanation tonometry. An increased dIOP (greater than 2 mm Hg) was detected in only 22% of Graves' patients [who had a mean dIOP of 3.5 +/- 1.6 (+/- SEM) mm Hg]. dIOP did not correlate with age, sex, age at disease onset, duration of disease, mode of antithyroid treatment, or thyroid function testing at the time of examination. Mean Hertel exophthalmometry measurements in patients with a dIOP greater than 2 mm Hg were 22.0 +/- 2.9 mm compared with 18.4 +/- 3.7 mm in those with a dIOP less than 2 mm Hg (P less than 0.027, by Wilcoxon rank sum test). Only 58% of patients with increased dIOP had clinical exophthalmos, but all had other evidence of Graves' eye disease. Computed tomographic scanning revealed significant proptosis and/or orbital muscle involvement in all of the patients with increased dIOP.