In a previous study we showed that subjects with abnormal binocular vision either lacked or had a deficient ability to adapt their oculomotor systems to a prism-induced heterophoria. In this study we report on the results of an experiment designed to establish whether the adaptation ability improves in patients who are successfully treated, by means of orthoptics, for convergence insufficiency. In all seven patients examined, we found an improved ability to adapt. Whereas at the beginning of the experiment only two patients fell within the 95% confidence limits of the norm, at the end five patients fell within these limits. These results demonstrate that the success of orthoptics in relieving symptoms is associated with an improved ability to adapt to prism-induced heterophoria.