Since increasing numbers of patients with connective tissue diseases are taking chloroquine and its derivatives, there is an urgent need for a simple, reasonably fast method of screening for toxic effects on the retina. Measurement of the critical flicker fusion frequency is one technique for assessing retinal function. Its screening value was assessed in a study of nine patients with bilateral visual field defects and visual acuity of 6/12 or better after various periods of chloroquine therapy and nine healthy subjects matched for age and sex who had never taken chloroquine. The control subjects' eyes consistently demonstrated the normal pattern previously described, whereas 16 of the 17 eyes tested in the patients showed one of two abnormal patterns of response.