BACKGROUND Besides decentration of the graft/host trephination and "horizontal torsion" "vertical tilt" is an important factor for reduced visual outcome after penetrating keratoplasty (PK). The purpose of this study was to evaluate the time course of vertical tilt in absolute value and direction and to correlate it with functional results after PK. METHODS Fifty patients each (20 primary dystrophies, 30 keratoconus) underwent nonmechanical trephination (NMT) (excimer laser MEL60, Aesculap-Meditec, Heroldsberg, Germany) or mechanical motor trephination (MT) (Geuder, Heidelberg, Germany) in penetrating keratoplasty. All procedures (7.5 mm in dystrophies, 8.0 mm in keratoconus, 8 orientation teeth in NMT, double-running 10-0 nylon suture) were performed by one surgeon (GOHN). At a postoperative gate of 6 weeks, 6 months, before partial suture removal and after complete suture removal, corneal topography analysis (TMS-1, Tomey, Tennenlohe, Germany) was performed. After a Gram-Schmidt-orthogonalization corneal topography height data of 25 noncentric rings in 256 hemimeridians were decomposed into Zernike components of radial order n = 16 in the sense of minimizing the root mean square error. The tilt of the surface relative to the videokeratoscope axis was calculated from the Zernike components Z1(1) and Z1(-1). The meridional power at the cardinal meridians was derived from all parabolic Zernike terms. Tilt and the difference between both meridians of the Zernike representation (ZA) were correlated with the results of Zeiss keratometry (KA). Simulated Keratometry (SimK) of the TMS-1, subjective refraction (RZ) and best-corrected visual acuity. RESULTS After NMT, vertical tilt of the graft was 3 degrees without significant change over time. Following MT, an equivalent time course could be observed before partial suture removal. However, after complete suture removal, a significant increase of the tilt was measured to 5 degrees (p = 0.02). No significant difference could be detected comparing keratoconus and Fuchs' dystrophy both in NMT and MT. The direction of the vertical tilt component piled up to the hemimeridian defined by the knot of the first running suture. At all postoperative follow-up examinations, the ZA of the Zernike decomposition showed a good correlation to the RZ, whereas the KA and the SimK did not. At the end of the follow-up, best-corrected visual acuity after NMT was 2 decimal lines better than after MT. CONCLUSIONS The Zernike decomposition of topographic height data is a suitable tool for extraction and quantifying vertical tilt of the graft following penetrating keratoplasty. In contrast to conventional keratometry with its 4-point measurement, a decomposition of topographic height data into orthogonal polynomials enables a detection of both cardinal meridians even in corneas with a high degree of local irregularities.