Sixteen patients enrolled in the PERK study were evaluated by fluorophotometry 24 hours or six months following radial keratotomy. A comparison of eyes operated and not operated upon showed that endothelial permeability was not significantly altered 24 hours and six months after surgery. Aqueous humour flow rates and anterior chamber elimination coefficients were significantly higher 24 hours after surgery in the eyes operated on than in those not operated on. Six months after surgery there was no longer a significant difference in these factors. The increase in aqueous humour flow rates 24 hours after surgery may represent a subclinical breakdown in the blood-aqueous barrier.