There are few reports in the literature that deal with the results of total knee arthroplasty in the younger patient. The present study was undertaken to evaluate the results of total knee arthroplasty in patients under the age of 55 years. Ninety-three knee arthroplasties in 62 patients were performed between 1974 and 1982. The preoperative diagnosis was rheumatoid arthritis in 76 knees and osteoarthritis in 17 knees. The total condylar knee prosthesis was used in all knees. Defects in the tibial plateau were noted in 13 knees, and supplemental support was required. Ninety knees (17 with osteoarthritis and 73 with rheumatoid arthritis) were available for follow-up study at a mean of 6.1 years. The knee rating for the entire group was 87.1. The subgroup scores for the osteoarthritis and rheumatoid arthritis groups were also 87.1. Roentgenographic evaluation revealed a lucency rate of 30%. Two implants were found to have global radiolucencies and were considered loose. Survivorship analysis resulted in a cumulative survivorship rate of 96% at ten years for the entire group. These results are comparable to the long-term results of total knee arthroplasty in the older patient and better than the results of total hip arthroplasty in the younger patient. Total knee arthroplasty in the younger patient is a reliable and durable procedure but should be used with caution despite the excellent results reported here.