In this study the Reichert non-contact tonometer (NCT II) was compared with the Goldmann applanation tonometer (GAT) in 25 subjects (50 eyes; pressure range 9-17 mmHg). The relationship NCT/GAT fulfilled the conditions required for legal verification. Marked differences between the readings of both instruments can be caused by anomalous central corneal thickness, because non-contact tonometry is influenced more than conventional applanation tonometry by the corneal thickness. Starting from a central thickness of 0.51 mm, the NCT is assumed to underestimate the actual intraocular pressure in eyes with thinner and to overestimate it in eyes with thicker cornea by 1 mmHg per 0.01 mm difference of corneal thickness. This value changes with individual corneal tissue qualities and is not valid especially in presence of corneal edema.