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To quantitatively investigate the time course of changes in corneal irregular astigmatism and their relation to best spectacle-corrected visual acuity (BSCVA) in eyes with keratoconus. Subjects were 47 patients (57 eyes) with keratoconus who had undergone videokeratography examinations at least twice, with an interval of 1 year or longer between each examination. By means of Fourier series harmonic analysis, topography data were separated into spherical component, regular astigmatism, asymmetry, and higher order irregularity. In eyes that showed apparent progression on color-coded maps, the yearly rate of change was significantly positive for spherical component (P = 0.03) and higher order irregularity (P = 0.006). In eyes with apparent progression, the yearly change rate of BSCVA was significantly positive (P = 0.001), but not in the eyes without apparent progression (P = 0.1655). In eyes with progression on maps, the yearly change rate of BSCVA was significantly correlated with the change rate of higher order irregularity per year (r = 0.770, P = 0.0004). The results indicated that progression of keratoconus is associated with deterioration of BSCVA, which is in turn significantly correlated with increases in corneal higher order irregularity. Jpn J Ophthalmol 2007;51:265–269 @ Japanese Ophthalmological Society 2007