To find out how large the total diameter of an open-loop intraocular lens (IOL) should be, we studied 90 consecutive implantations of a one-piece poly(methyl methacrylate) IOL with a 7 mm biconvex optic and a total diameter of 10 mm (Adatomed 75 ST). The lens design made implantation simple, even through a small capsulorhexis. In 85% of the cases, the IOL remained well centered. One case with asymmetric placement of the IOL showed a decentration of more than 1 mm. In 14% of the eyes slight decentrations occurred because the capsular bag was too large. Contact between the iris and IOL was found in 15%. In 4% there were folds in the posterior capsule, and in 35% there was a small space between the IOL and the posterior capsule because the capsular bag was too large. Although the ST lens has a small total diameter, it remained at a distance from the iris in most cases. In-the-bag implantation through a continuous central capsulorhexis is mandatory to avoid secondary displacement; nevertheless, slight decentrations cannot be excluded. For a large capsular bag, the total diameter of an IOL should exceed 10 mm. Clinical results, cellular reaction on the lens surface (26.6%), biocompatibility, and visual performance are comparable to those of other well-approved poly(methyl methacrylate) IOLs.