The authors aimed to investigate the accuracy of the 15^ base in prism test (BIPT) in detecting amblyopia in anisometropic patients. This was a retrospective study in which the 15^BI was placed before the right eye followed by the left eye whilst fixating a light target and comparing the responses of either eye for fixation preference; 152 children met the inclusion criteria. Median age was 5.4 years; 102 had amblyopia with two or more lines visual acuity difference between eyes. All had heterophoria and stereopsis. Overall sensitivity of the 15^BI to detect amblyopia was 34.3% with specificity of 88%, positive predictive value of 85.4% and negative predictive value of 39.6%. The authors conclude the 15^BI is a poor test for detecting amblyopia in anisometropic patients but in view of this being a retrospective study, further research is warranted.