OBJECTIVE To determine the immediate effect on corneal thickness of cataract extraction using phacoemulsification. METHODS Prospective consecutive nonrandomized humans undergoing standard small-incision phacoemulsification and intraocular lens insertion. METHODS Corneal thickness was measured immediately preoperative and postoperatively in 58 consecutive patients undergoing phacoemulsification in the absence of other ocular abnormalities. Corneal thickness was determined centrally and in the midperiphery of four quadrants. The last 17 eyes were also evaluated for the effect of pressure from a Honan balloon on corneal thickness. METHODS Faculty practice in medical school and university hospital. The cornea thinned in all five of the measured locations, with statistical significance being reached only in the four midperipheral quadrants. Pressure from the Honan balloon resulted in a negligible increase in corneal thickness preoperatively. CONCLUSIONS Corneas were found to have thinned statistically significantly immediately after phacoemulsification; however, the degree of thinning has doubtful clinical significance and does not represent a significant dehydration of the cornea during the surgical procedure.