A formula for predicting aphakic refraction from axial length and keratometry measurements was derived from 120 cases, using linear regression analysis, as follows: Psp = 80.4 - 1.65L -0.7K, where P indicates aphakic spectacle power in diopters; L, axial length in millimeters; and K, result of keratometry in diopters. The prediction accuracy of this simple formula was compared with that of a complex theoretic formula based on geometric optics in 205 additional cases. The prediction accuracy of the theoretic and linear regression-derived formulas was virtually identical despite their differences in complexity and derivation. The differences and similarities between theoretic-derived and regression-derived aphakic and pseudophakic prediction formulas are discussed.