Opening wedge high tibial osteotomy (OWHTO) is a recently described procedure for medial compartment arthritis of the knee in the active, younger population. Despite having a number of advantages over the traditional closing wedge high tibial osteotomy (CWHTO) a potential complication of OWHTO is a high rate of delayed- and non-union. This study reports the occurrence of delayed- and non-union following OWHTO for medial compartment arthritis of the knee. Questionnaires were sent to all current members of the Australian Knee Society (n=45), a special interest group of the Australian Orthopaedic Group. Surgeons were asked primarily to indicate how many OWHTOs they had performed, and how many of these had progressed to union, delayed-union and non-union. All 45 questionnaires were returned, with 21 surgeons (47%) performing OWHTOs. A total of 188 OWHTO cases were reported, of which 182 were complete. Of these complete cases 167 (91.8%) were classed as united, 12 (6.6%) delay-united and 3 (1.6%) non-united. The results of this study demonstrate that the rate of delayed- and non-union following OWHTO for medial compartment arthritis of the knee is relatively low and comparable to that reported for traditional CWHTO.