Valgus tibial osteotomy is a well-recognized procedure for medial compartment arthritis of the knee. The femorotibial angle must be adequately realigned; most unsatisfactory outcomes result from undercorrection. This study of 27 patients (38 knees) with a valgus tibial osteotomy for unicompartmental arthritis of the knee emphasizes the importance of correct alignment. Alignment outside the acceptable range did accelerate deterioration in these patients, followed for an average of 53 months. In 25 of the 38 knees (66%) results were good to excellent. An alignment of 6 degrees or more of valgus produced an excellent to good result in 21 of 23 knees (91%). Alignment of less than 6 degrees of valgus yielded excellent to good results in 4 of 15 knees (27%). Although at the end of one year, 31 of the 38 knees were rated as excellent to good, these knees progressively deteriorated. The knees in 6 degrees or more of valgus seldom deteriorated while those in less than 6 degrees of valgus deteriorated more frequently. A valgus tibial osteotomy is an excellent procedure provided the tibiofemoral angle is realigned to 6 degrees or more of valgus.