High tibial osteotomy is an accepted treatment for unicompartmental osteoarthritis of the knee. Conventional osteotomy can be a demanding procedure with potential for complications. Opening wedge high tibial osteotomy using an external fixator is an alternative that may have advantages in comparison with classic methods. The aims of the current study were to determine if opening wedge osteotomy using hemicallotasis techniques is safer than, and the outcome comparable with that of, conventional techniques. Seventy-six high tibial osteotomies were performed in 65 patients for primary osteoarthritis. The mean age of the patients was 54.8 years (range, 36-70 years). The mean followup was 6 years. The only serious complication occurred in one patient, who had chronic osteomyelitis develop 2 years after surgery. There were no neurologic or vascular complications. The authors think this technique is safer than conventional techniques. Survivorship at 5 and 10 years was 89% and 63%, respectively. The mean knee score in osteotomies was 26.6 (maximum possible score, 48). The outcome is comparable with, or better than, that of other techniques for osteotomy. Subsequent knee replacement, in cases requiring conversion, was straightforward. The mean score in knees that had osteotomies that were converted to total knee replacements was 33.7.