Diurnal changes in corneal topography have been documented to be greater in eyes following radial keratotomy that in normal eyes. To determine whether substantial changes of this nature result from corneal incisions in general or are a specific complication of radial keratotomy, we examined 12 corneas (10 patients) after penetrating keratoplasty for diurnal changes, using a computerized corneal topographic analysis system. Diurnal changes in keratometry readings, central corneal thickness, intraocular pressure, and refraction also were analyzed. From morning to evening, central corneal curvature flattened by an average of 0.74 +/- 0.18 diopters in eight eyes (67%), and steepened by an average of 2.00 +/- 0.53 D in four eyes (33%). Diurnal changes in central corneal thickness were correlated with diurnal changes in central corneal curvature (r = 0.63, P less than .05). These data suggest that changes in corneal thickness over the course of the day may partially account for the changes in corneal curvature demonstrated to occur following penetrating keratoplasty.