High tibial osteotomy has become a common surgical treatment for osteoarthritis. From July 1976 to July 1985, we performed high tibial osteotomies on 99 knees in 91 patients using Bauer's and Koshino's wedge osteotomy. We fixed bone with two Steinman pins. Of these patients, more than 5 years had elapsed since operation in 51 with 57 operated-on knees, and we did a follow-up on them with respect to operating technique and prognosis. Four patients had died and 12 were unavailable. The period of follow-up on the remaining 40 knees in 35 patients was, on an average, 7 years and 3 months. We made evaluations based on the Yokohama City University evaluation table of knee joints. After including patients'judgments, we classified knees into 4 grades; excellent, good, acceptable and failure. The total results were as follows: good, 22 knees; acceptable, 16 knees; failure, 2 knees.<BR>These results were worse than those over a shorter period. Among 6 knees operated on with hemi-open, 2 had delayed union. The causes of delayed union on 5 other joints were the following; in 3 joints the fit of the inside bone cortex was inadequate, so that a dent occurred; in one joint there was a superficial infection; and in one joint the Steinman pin inserted was not positioned well. The cases which had a FTA showing 164°-174° had good results. Excessive osteotomy met with better results than insuffient osteotomy.<BR>We also examined the proceeding of FTA on patients with non-union peroneal bone at this time. Some of those with valgus or varus deformities were getting worse than those with good-union peroneal bone. It is important for obtaining stable results with high tibial osteotomy to perform optimum osteotomy and then promptly accomplish and support bony-union.