A modified focusing method for measuring simultaneously the refractive-index profile and the cross-sectional geometry of an optical fiber preform is presented. In this method the preform is immersed in indexmatching liquid, and collimated light is made to impinge on the preform laterally. The intensity distributions of the transmitted light are detected on two planes inside the preform core. From the recorded light intensity distributions, the deflection function is calculated. After the preform has been rotated through 180 degrees and the necessary data have been collected, a numerical inversion is performed to obtain the 2-D index profile. In this paper, the principle, the experimental setup, and results are described. The sensitivity of the method to measurement errors and the accuracy of the inversion formula were investigated by computer simulations. It was found that the error of the index profile measurement was <1%.