The upper three-quarters of the fibula is commonly used as a non-vascularised autograft. Subsequent to this isolated weakness of extensor hallucis longus may occur. We have studied 26 patients in whom the upper and middle thirds of the fibula had been harvested as a graft through Henry’s posterolateral approach. Isolated weakness of extensor hallucis longus was found after operation in ten patients but not in the remainder. EMG and nerve-conduction studies confirmed injury of the nerve to extensor hallucis longus in those with weakness. We dissected 40 cadaver limbs and found that those in which the nerve to extensor hallucis longus ran close to the fibular periosteum were at risk. The injury is mostly incomplete and recovery occurs within four to six months