A clinical study of 105 upper tibial osteotomies was undertaken to investigate the incidence, pathology and outcome of perioperative neurological deficit. Motor weakness and/or sensory deficit occurred in 21 patients (20%) and in half of these the deficit was permanent. For descriptive purposes the fibula was divided into four zones. The occurrence of a neurological deficit was clearly related to the level of the fibular osteotomy. An anatomical explanation is proposed for this complication, based on cadaveric studies. Due to unacceptably high levels of complications it is recommended that the fibular osteotomy should not be performed in zones II and III (from just below the fibular head to 15 cm distal to this level).