The purpose of the current study was to review results of primary constrained condylar knee arthroplasty in elderly patients with genu valgum deformity. The hypotheses were: (1) constraint has no adverse effects in elderly patients; (2) treating deformity with a constrained condylar knee prosthesis in lieu of lateral ligament release avoids morbidity, particularly peroneal nerve palsy and flexion instability; and (3) press-fit noncemented stem extensions enhance fixation of the cemented core components and are not prone to loosening. Between 1988 and 1993, 44 consecutive primary Constrained Condylar Knee prostheses were implanted in 37 patients (average age, 72.7 years) with an average valgus angle of 17.6 degrees. Indications for the Constrained Condylar Knee implant were: elderly patients with genu valgum deformity and medial collateral ligament incompetence. Outcome was assessed prospectively using the Hospital for Special Surgery and Knee Society scoring systems; followup was by independent observer. Clinical and radiographic followup (average, 7.8 years) was available for 28 knees (26 patients). The Hospital for Special Surgery score improved from 52.2 to 89.6 points. The average Knee Society score and functional scores improved from 27.4 and 32.4 points to 95.2 and 67.2 points, respectively. At followup, the average alignment based on anteroposterior radiographs obtained with the patient weightbearing was 5.3 degrees. No radiographic loosening, prosthetic failures, peroneal nerve palsies, or flexion instability occurred. No failures occurred in the 11 patients (16 knees) who died before the latest followup. To the authors' knowledge, this is the largest reported series with the longest reported followup of patients with primary Constrained Condylar Knee prostheses. The use of the Constrained Condylar Knee prosthesis for elderly patients with low physical demands with genu valgum resulted in significant pain relief and improved function.