The clinical performance of a portable ultrasound pachometer was compared with a digital, 'optical' pachometer attachment of a contact specular microscope. The two separate methods were used to assess central corneal thickness (CCT) in 214 extended-wear soft contact lens subjects over a cumulative 33 month period. The two test methods did not correlate well (r = 0.56). Mean ultrasound CCT was 0.542 mm +/- 0.004 SEM; mean CSM 'optical' attachment CCT was 0.562 mm +/- 0.004 SEM (significantly different by t-test; p < 0.05). A two-way anova (each CCT method by immediate duration of extended lens wear, and by duration in study) revealed an apparent underestimation of central corneal thickness by the ultrasound pachometer. While this presumed ultrasound bias or error can be explained by concurrent morphological and physiological changes associated with long-term contact lens wear, other methodological errors could have influenced the results. Pending additional study, physiologically stressed corneas might be better evaluated by some method of corneal thickness measurement other than ultrasound.