The interpretation of the edrophonium (Tensilon) test in the diagnosis of acquired strabismus caused by myasthenia gravis has been problematic because of poorly defined endpoints and unknown sensitivity and specificity. We evaluated the endpoint criteria, dynamics, sensitivity, and specificity of binocular alignment in response to edrophonium by using the Hess screen test in ten normal control subjects, 12 nonmyasthenic patients with acquired strabismus, and in ten patients with acquired strabismus caused by ocular myasthenia gravis. A positive response to the edrophonium-Hess screen test was defined as a 50% or greater reduction in the strabismic deviation at the fixation point associated with maximum deviation within one minute of edrophonium infusion. All myasthenic patients had a 50% or greater reduction in the initial deviation within one minute of edrophonium infusion. Myasthenic patients had a statistically significant reduction in the average deviation up to 150 seconds after edrophonium infusion (P < .05 for all time periods). In contrast, with or without edrophonium infusion, control subjects had a purely horizontal fluctuation in binocular alignment of less than or equal to 2 degrees for the entire four-minute period after edrophonium infusion. None of the 12 nonmyasthenic patients tested positive to the edrophonium-Hess screen test. According to the criterion of positive response as was defined in this study, the test had a high sensitivity and specificity in this sample. These results suggest that clearly defined endpoint criteria make the edrophonium-Hess screen test a sensitive and specific quantitative study for the diagnosis of acquired strabismus caused by myasthenia gravis.