From a case-control study of out-patients attending an ophthalmic clinic no relationship (relative risk = 1) was found between refractive error and ocular tension. The deconfounded effect of refractive error on open angle glaucoma (field loss) increased at higher levels of myopia (relative risk = 1.18; refractive error -0.2) The combined effects of myopia and severe ocular hypertension were found to be synergistic (interactive risk ratio = 2.2).