This study compares the results of 16 total knee arthroplasties after a spontaneous osseous ankylosis and 14 total knee arthroplasties after a takedown of formal knee fusion. This series is a collection of patients who have either ankylosis or arthrodesis because of previous pyogenic or tuberculous infection. There were 18 women and 12 men. The age at operation ranged from 30 to 62 years (average, 42.9 years). The duration of ankylosis was longer in the patients in the spontaneous ankylosis group (19.7 years) than in the patients in the formally fused knee group (11.3 years). The duration from prior infection to arthroplasty was 12.4 years (range, 6-22 years) in the patients in the formally fused group and 20.4 years (range, 7-39 years) in the patients in the spontaneous ankylosis group. The original diagnosis was tuberculous arthritis in 14 knees and pyogenic arthritis in 16 knees. The average follow-up was 5.3 years (range, 5-6 years). Gender, age, diagnosis, and follow-up period were comparable in both groups. The preoperative Hospital for Special Surgery Knee Score was 60 in both groups, and the average knee score in both groups at the final follow-up was 73 (range, 67-75). Comparison of the patient's preoperative and postoperative ambulatory status revealed functional improvement in both groups. The average range of active flexion in both groups was 75.8 degrees (range, 70 degrees-95 degrees). The extension lag in both groups averaged 9 degrees (range, 0 degrees-20 degrees). The range of motion was approximately the same in both groups. The angular deformity was corrected to 0 degrees to 10 degrees of valgus angulation in all knees. There was no revised prosthesis for loosening at the final follow-up. Complications included skin edge necrosis (53.3%), pyogenic infection in 2 patients, and a quadriceps tendon rupture in 1 patient.