Unicompartmental knee arthroplasty was conceived for the treatment of osteoarthritic knees with deterioration of either the medial or lateral compartment of the knee. The Unicondylar prosthesis was used in 19 medial and 5 lateral compartment arthroplasties. A two to 4 year follow-up evaluation of these patients was somewhat disappointing as only 5 knees could be rated excellent and 6 rated a good result. The best results were seen in the lateral compartment arthroplasties. Such deformities may be the only future indication for the use of this operation as these knees do not do well when treated by tibial osteotomy. However, when only the medial compartment is involved, osteotomy may still remain the treatment of choice.