The aim of this study was to evaluate the association between primary inferior oblique over action and astigmatism along the axis on the overacting muscle. The authors explored the postoperative change in axis of astigmatism and cylinder power in a retrospective study of 118 patients (mean age 11.31 ±10.59 years). Sixty subjects had no inferior oblique over action, 41 had bilateral inferior oblique over action and 17 had unilateral inferior oblique over action. The median over action was +3. Sixty-two percent of eyes had mild astigmatism <1D, 35% with moderate astigmatism of 1-3D and 3% with high astigmatism >3D. For those with no / mild astigmatism, postoperative there was significant median increase in with-the-rule cylinder power of 0.28D. Those with bilateral inferior oblique over action had significant increase of cylinder power along the 90 degree axis on -0.45D mean change. This was not observed in the unilateral over action group but analysis was limited by small numbers in this group. The results show that eyes with inferior oblique over action do not have increased prevalence of astigmatism postoperatively.