The aim of this study was to evaluate the accuracy of ultrasound biomicroscopy (UBM) preoperatively and postoperatively in patients having primary horizontal muscle surgery. Sixteen eyes of 15 patients with concomitant horizontal strabismus were evaluated; mean age 21.1 ±3.3 years, five esotropia and 11 exotropia. Medial rectus was operated in 14 eyes and lateral rectus in 16 eyes. Preoperative UBM distance of the medial rectus insertion from limbus was 4.9-5.9mm (mean 5.3 ±0.3mm) and 5.9 to 8.3mm (mean 6.8 ±0.7mm) for the lateral rectus. Preoperative distance measured directly during surgery of the medial rectus insertion from limbus was 5-6mm (mean 5.6 ±0.3mm) and 6.5-8mm (mean 7.1 ±0.4mm) for the lateral rectus. For all medial and 14/16 lateral rectus measurements, these were within ±1mm with no significant differences found. For two lateral rectus measurements, differences were 1.3 and 1.5mm. Postoperative accuracy was less as these were found to be outside ±1mm or, alternatively, the muscle (always lateral rectus) could not be visualised. The authors conclude that UBM was unreliable in detecting the muscle insertion after recession with only five of 16 muscles detected. Measurements were more reliable in detecting insertions after resection. UBM can accurately predict preoperative extraocular muscle insertions for untouched horizontal recti. Postoperative maximum distance accurately measured was 11.2mm from the limbus.