PURPOSE: To compare the central corneal thickness measured by online optical coherence pachymetry (OCP) and ultrasound pachymetry in normal cornea. METHODS: Forty-eight right eyes of 48 consecutive patients were enrolled in this prospective study. Central corneal thickness measurements were taken intraoperatively with the online OCP and ultrasound pachymeter before flap creation. The precision repeatability, intraclass correlation coefficient, and correlation of variation for each instrument was calculated. Intraclass correlation coefficient was based on Bland-Altman plot of differences between instruments. RESULTS: Mean central corneal thickness for ultrasound pachymetry and online OCP was 559.45±33.05 μm (range: 475.50 to 650.50 μm) and 521.19±28.97 μm (range: 447.50 to 606.00 μm), respectively. The precision (repeatability) was 7.86 μm and 9.47 μm, respectively. The intraclass correlation coefficient for the ultrasound pachymeter was 0.997 (95% CI, 0.993-0.998) and 0.993 (95% CI, 0.988-0.996) for the online OCP. The coefficient of variance was 0.50% and 0.66%, respectively. The mean difference between ultrasound pachymetry and online OCP was 38.26±9.96 μm. The limits of agreement were: upper=58.19 μm (95% CI, 53.30-63.07 μm) and lower=18.34 μm (95% CI, 13.45-23.22 μm). CONCLUSIONS: Online OCP can be used as a reliable alternative to ultrasound pachymetry as both instruments gave highly repeatable measurements of central corneal thickness. However, measurements using the OCP were consistently lower than those with the ultrasound pachymeter; therefore, the two techniques should not be used interchangeably. Further studies are needed to determine the implications of thinner central corneal thickness measured with online OCP as well as to resolve the systematic differences in measurements by these pachymetric technologies.