Purpose: To develop new methods for quantifying diplopia and test their validity, test-retest reliability, and responsiveness to treatment. Design: Prospecti ve case series. Participants: Adult patients with diplopia in 2 tertiary referral practices. Diagnosis included cranial nerve palsies, thyroid eye disease, supranuclear palsies, and ocular myasthenia gravis. Methods: The new diplopia questionnaire and cervical range of motion (CROM)-diplopia examination were tested in a 15-patient validity study against the standard diplopia field performed on the Goldmann perimeter (each scored 0 no diplopia to 100 Ca nstant diplopia in all fields). The test-retest reliability of the CROM diplopia examination was determined in 27 patients. The responsiveness to treatment of the diplopia questionnaire and the CROM examination was tested in 15 patients who underwent strabismus surgery or received systemic prednisone for ocular myasthenia gravis. Main Outcome Measures: Validity was evaluated by the intraclass correlation coefficient (ICC) compared with the standard Goldmann diplopia field. Test-rest reliability was evaluated by the ICC. Responsiveness to treatment was evaluated by comparing pretreatment and posttreatment scores. Results: There was excellent agreement between the diplopia questionnaire score and Goldmann diplopia field score (ICC, 0.90) and between the CROM diplopia examination score and Goldmann diplopia field score (ICC, 0.88). Intertester reliability of the CROM diplopia examination score was excellent (ICC, 0.87; 95%confidence interval, 0.74-0.94). Postintervention scores were significantly lower than baseline (P=0.0003 for CROM and P=0.0001 for the questionnaire), with a median postintervention improvement of 48 for the CROM diplopia score (quartiles 12-68) and 36 for the diplopia questionnaire (quartiles 20-52). Conclusions: Our new diplopia questionnaire and new CROM diplopia examination are valid, reliable, and responsive measures of diplopia seve rity. The tests are easy to administer and require less expensive and less cumbersome equipment than previously available. Purpose: To develop new methods for quantifying diplopia and test their validity, test-retest reliability, and responsiveness to treatment. Design: Prospecti ve case series. Participants: Adult patients with diplopia in 2 tertiary referral practices. Diagnosis included cranial nerve palsies, thyroid eye disease, supranuclear palsies, and ocular myasthenia gravis. Methods: The new diplopia questionnaire and cervical range of motion (CROM) -diplopia examination were tested in a 15-patient validity study against the standard diplopia field performed on the Goldmann perimeter (each scored 0 no diplopia to 100 Ca nstant diplopia in all fields). The test-retest reliability of the CROM diplopia examination was determined in 27 patients. The responsiveness to treatment of the diplopia questionnaire and the CROM examination was tested in 15 patients who underwent strabismus surgery or received systemic prednisone for ocular myasthenia gravis. Main Outcome Measures: Validity was evaluated by the Test-rest reliability was evaluated by the ICC. Responsiveness to treatment was evaluated by comparing pretreatment and posttreatment scores. Results: There was excellent agreement between the diplopia questionnaire score and Goldmann diplopia Interaction of the CROM diplopia examination score was excellent (ICC, 0.87; 95% confidence interval, 0.74-0.94). Postintervention scores were significantly lower than baseline (P = 0.0003 for CROM and P = 0.0001 for the questionnaire), with a median postintervention improvement of 48 for the CROM diplopia score (quartiles 12-68) and 36 for the diplopia questionnaire (quartiles 20- 52). Conclusions: Our new diplopia questionnaire and new CROM diplopia examination are valid, reliable, and responsive measures of diplopia seveirity. The tests are easy to administer and require less expensive and less cumbersome equipment than previously available.