OBJECTIVE A prospective randomised clinical trial was set up to compare the effect of hydroxypropyl methylcellulose (Ocucoat) and sodium hyaluronate (Healonid) on pupil size and reactivity following their use in cataract surgery. METHODS Pupil measurements were recorded before and 6 weeks after surgery. RESULTS There was no significant difference between the two groups with respect to pupil size (p = 0.69, Mann-Whitney U test) nor with respect to reactivity (p = 0.99, Fischer's exact test). Ninety six per cent of the surgery was performed using phacoemulsification. CONCLUSIONS This trial suggests that both viscoelastic materials have similar effects on the pupil after their use in cataract surgery.