A two-phase process evaluating the postoperative diplopia test is presented. A standardised operating procedure was used for conducting all postoperative diplopia tests for this study which is described in the paper. Phase one focused on reliability, test-retest and interobserver. Adults with constant manifest strabismus and suppression were eligible to participate and were prospectively recruited. Exclusion criteria included having a greater vertical deviation, no perception of light in the strabismic eye and evidence of binocular potential. Phase two involved a retrospective examination of patients who had undergone horizontal muscle surgery over a specified 12-month period. Details of the surgery, pre- and postoperative strabismus and symptom reporting were extracted. Thirty-nine participants were recruited to phase one. Group 1 for test-retest assessed 20 participants with eight demonstrating a range of variability for test-retest and nine at near and 10 at distance demonstrating variability for interobserver. No correlation analysis was performed. A total of 39 patients met the inclusion criteria for phase two, of which 20 had postoperative follow-up to contribute to the predictive analysis. The two patients with persistent diplopia beyond three-months postoperative had both identified diplopia on the postoperative diplopia test. The authors highlight the high number of cases of variability for both test-retest and interobserver despite using a standardised operating procedure. They concluded that the postoperative diplopia test does not have the power to be predictive of postoperative diplopia. A number of limitations of this are highlighted including the small sample size and convenience sample. This study highlights that clinical decisions should not be made solely based upon the postoperative diplopia test.