OBJECTIVE This retrospective study was done to identify risk factors for cataract formation and cataract extraction after penetrating keratoplasty to determine whether a combined procedure (simultaneous cataract extraction with keratoplasty) or penetrating keratoplasty alone should be performed. METHODS Variables from 342 phakic eyes having undergone penetrating keratoplasties were studied with univariate and multivariate analyses, including diagnosis, race, sex, age, preoperative lens opacities, preoperative vision, and length of follow-up. RESULTS For cataract formation, age was the only independent risk factor found by multivariate analysis (P = 0.0001). For cataract extraction after penetrating keratoplasty, independent risk factors included age, sex, diagnosis, and preoperative lens opacities (P < or = 0.03). For example, the probability of a 65-year-old patient with Fuchs dystrophy requiring a cataract extraction within 5 years of keratoplasty is 81%. CONCLUSIONS The likelihood of cataract formation and cataract extraction subsequent to penetrating keratoplasty increases greatly after 50 years of age, regardless of the diagnosis leading to the need for keratoplasty. The need for cataract extraction also is increased for female patients, for patients with Fuchs dystrophy, and for those with early preoperative lens opacity.