To study the intrasession and intersession repeatability of the Pentacam system (Oculus Inc.) in measuring the posterior corneal shape. School of Optometry, Hong Kong Polytechnic University, Hong Kong, China. The posterior corneal shape in 1 eye (randomly selected) was measured with the Pentacam system. Three consecutive readings were taken for intrasession repeatability analysis. Measurements were repeated 1 to 2 weeks later for intersession repeatability analysis. The anterior best-fit sphere (BFS) and posterior BFS at the 5.0 mm and 8.0 mm zones, as well as the elevation of the posterior cornea at these 2 zones, were compared. The anterior and posterior BFS demonstrated good intrasession repeatability (Cronbach alpha > or = 0.997; intraclass correlation coefficients > or = 0.990). No significant difference was found between the 2 visits. The 95% limits of agreement of posterior BFS were 52.3 to -41.7 microm at the 5.0 mm zone and 66.1 to -60.9 microm at the 8.0 mm zone. The elevation was similar between the 2 visits. The 95% limits of agreement were 4.27 to -5.29 microm at the 5.0 mm zone and 5.33 to -5.77 microm at the 8.0 mm zone. Pentacam provided good performance in assessing the posterior cornea. To improve the intersession repeatability, it is suggested that 3 consecutive readings, rather than 1 image, be taken to generate an average BFS for analysis. When comparing the posterior corneal elevation between sessions, an average BFS generated from the first visit should be used for elevation calculation.