The clinical performance of a portable ultrasonic pachometer was compared with a modified optical pachometer on a group of normal subjects. The mean central corneal thickness according to optical pachometry using the "edge-to-edge" method of optical doubling was 0.553 mm (SD=±0.022). The mean central corneal thickness using the ultrasonic device was 0.506 mm (SD=±0.039). The differences between the means was statistically significant (at 1% level of probability). Based on the light distribution within the corneal optical section viewed during optical pachometry a hypothesis is derived supporting the notion that optical pachometry tends to overestimate corneal thickness rather than ultrasonic pachometry underestimating corneal thickness.