In a polycentric study, the influence of some of the most common variables capable of influencing the reliability of prekeratotomy pachymetry has been evaluated. The fluctuation in a sequence of measurements from the same point on the cornea, with the patient in different positions (orthostatic and clinostatic), during progressive drying of the lacrimal film and before and after corneal marking have been evaluated. Three operators, working randomized, used the same computerized pachymeter (DGH 1000) to make the measurements. The data obtained in this way were statistically analyzed by the Statgraphics personal computer program. Some interesting and useful considerations about the more correct methods to run the test and its real accuracy have emerged.