A three-dimensional, in vivo method for determining the position of implanted intraocular lenses (IOLs) using an image-processing technique has been developed. Two images of the anterior segment of the eye with an implanted IOL were photographed with a Scheimpflug camera at slit-lamp angles of 90 degrees and 180 degrees. After geometrical correction, the contours of the original images were enhanced using binarization and curve-fitting techniques. These images afforded calculation of the angle of tilt of the IOL optic axis relative to a standard reference line which connected the center of anterior corneal surface curvature with the geometrical center of the pupil. The extent and direction of IOL decentration could also be shown. The IOL image in the posterior chamber was expressed as a wire-frame figure. This method provides several kinds of biometry in pseudophakic eyes.