Background and purpose High tibial valgus osteotomy (HTO) is a well-accepted treatment for medial unicompartmental osteoarthritis of the knee with varus alignment in relatively young and active patients. Controversies about the factors affecting survival of HTO still exist. We assessed preoperative risk factors for failure of closing-wedge HTO at long-term follow-up. Patients and methods A cohort of 100 patients with a mean age of 49 (24-67) years, who had closing-wedge HTO performed between January 1991 and December 1996, were analyzed retrospectively. A survival analysis was carried out according to the Kaplan-Meier method. Logistic regression analysis was used to assess the association between failure of the osteotomy and known potential preoperative risk factors. Results The probability of survival for HTO was 75% (SD 4%) at 10 years with knee replacement as the endpoint. Female sex and osteoarthritis of grade 2: 2 were identified as preoperative risk factors for conversion to arthroplasty 10 years after HTO. Interpretation Our findings suggest that ideal candidates for corrective osteotomy are men with symptomatic medial unicompartmental osteoarthritis of Ahlback grade 1, who, 10 years after surgery, have an almost tenfold lower probability of failure of HTO than women with more advanced osteoarthritis.