Eighty patients scheduled for primary total knee arthroplasty were randomized to instrumentation with custom cutting blocks or to conventional intramedullary and extramedullary instrumentation. Intraoperative data concerning operative time and blood loss were collected, and radiographic outcome was analyzed at 6 weeks postoperatively. The results indicated that mechanical alignment was adequate with the use of custom cutting blocks, and similar to that achieved by conventional instrumentation. The groups did differ significantly in rotation of the femoral component, with patients of the custom cutting blocks group displaying significantly less external rotation (1° external rotation) and more consistency in placement (Range 0°-2.5° external rotation) compared to conventional instrumentation (Mean 1.6° external rotation; Range 2.3° internal - 4.9° external rotation). Operative times were also measured to be shorter in the custom cutting blocks group.