The authors evaluate a novel assessment of botulinum toxin (BT) injection into the inferior oblique (IO) as a postoperative test in those with secondary IO overaction. This was a retrospective review of 23 injections for 18 patients using 1.25U of Xeomin BT into the IO under electromyographic (EMG) guidance. There were nine males and nine female patients with a mean age of 42.3 years (21-75). The main condition was superior oblique palsy in 15, and three with orbital fracture, retinal detachment with explant or IO inclusion syndrome. Deviation in primary gaze was 5.5 PD (SD3.5) preoperatively reducing to 1.8 PD (SD1.6) postoperatively. Deviation on contralateral side gaze was 13.3 PD (SD9.9) reducing to 5.2 PD (SD6.2) postoperatively at two weeks. Fourteen patients reported improvement in symptoms and went on to inferior oblique myectomy surgery. Two opted for repeat BT at three to four month intervals. One required a superior oblique tuck procedure. The remainder were treated conservatively. The main purpose was to evaluate BT as a postoperative diplopia test and the authors concluded it was useful as such although it is also viable for some patients as a long-term treatment option.