To compare the endothelial and clinical outcome of penetrating keratoplasty with corneas stored in organ culture for up to 12 days (5-12 days; group 1) or more than 21 days (21-24 days; group 2). Methods: We conducted a controlled double-masked trial. Storage durations were randomly assigned to the paired corneas, and endothelial cell density (ECD) was measured at the start and end of organ culture. Patients with a low rejection risk and preoperative ECD within the reference range were randomly assigned to 1 of the 2 groups and underwent an 8.25-mm penetrating keratoplasty (n=25 pairs). Follow-up at day 5 and months 1, 6, and 12 included central ECD, morphometry, graft transparency, visual acuity, pachymetry, and complications. The main outcome measure was the central ECD at month 12. Results: At the end of organ culture, ECD of the group 1 corneas was higher by 273 cells/mm 2 (95% confidence interval [CI], 178-368; PϽ.001). One year after penetrating keratoplasty, the group 1 ECD was still comparably higher by 227 cells/mm 2 (95% CI, 43-411; P=.02). Graft transparency, pachymetry, and complication rate did not differ at any time. In group 1, visual acuity was better at month 1. Conclusions: Shorter organ culture allows delivery of corneas with higher ECD. Recipients with ECD within the reference range and low rejection risk retain this initial benefit 1 year postoperatively. The higher endothelial cell capital may prevent or delay late endothelial failure, the leading cause of graft failure in these recipients. We therefore prefer short-term storage for such recipients.