We evaluated the intraobserver, interobserver, and intersession variability of the Humphrey model 580 ultrasonic pachometer. Twenty eyes (10 subjects) were measured in the intraobserver study. Forty-two eyes (21 subjects) were measured in the interobserver and intersession studies. Multiple analysis of variance revealed no statistical differences between observers or between right and left eyes. A marginally significant time (intersession) effect was found, with greater corneal thicknesses measured in the afternoon. The intraobserver variability averaged 6 microns, whereas the interobserver and intersession variabilities were approximately 11 microns and 14 microns, respectively. The relative simplicity, objectivity, and repeatability of ultrasonic pachometry should enable corneal thickness changes to be monitored in clinical practice on a more routine basis.