OBJECTIVE: The authors compared two methods, the Marcus Gunn test and the alternating light test, for detecting a relative afferent pupillary defect. DESIGN: A randomized, prospective clinical trial. PARTICIPANTS: Fourteen patients with unilateral optic neuropathy. INTERVENTION: The Marcus Gunn and alternating light tests were performed on each patient. MAIN OUTCOME MEASURES: The results of the Marcus Gunn and altemating light tests for detecting a relative afferent pupillary defect on the affected side. RESULTS: The Marcus Gunn test was able to identify the affected eye in only 8 of 14 patients, whereas the alternating light test correctly identified the affected eye in 13 of 14 patients. Results of the Marcus Gunn test were indeterminate in 4 of 14 patients and were incorrect in 2 of 14 patients. Results of the alternating light test were indeterminate in one patient and never incorrectly identified the affected eye. CONCLUSION: The alternating light test is superior to the Marcus Gunn test for detecting relative afferent pupillary defects.