The correction of axial deviation of the lower limbs in paediatric patients requires complete patient history and clinical examination. The correct approach to the deformity must consider:radiology,evolution,associated joint damage, neurologic diseases and surgical planning. Timing and choice of operation (osteotomy, assimetric epiphyseal distraction, hemiepiphysodesis, epiphysiodesis) are critical. Thirty-four paediatric patients with an average age of 10 years (range 2–18) were treated with a monolateral external fixator: 16 femoral osteotomies (nine post-traumatic, four congenital, one after radiotherapy for neoplastic diesease, one Ollier’s disease, one multiple exostosis) and 18 tibial osteotomies (eight congential, four post-traumatic, two multiple exostosis, two osteogenesis imperfecta, one neoplasm, one Ollier’s disease) were performed. The knowledge of normal physiologic values, angles and anatomical and mechanical axes are fundamentally important. In choosing which operation to perfom in patients with post-traumatic deviation, the controlateral limb, functional necessity, symptoms and possible compensation must be considered. External fixation appears to be necessary in the correction of lower limb deformities because of patient comfort in the femur, because it allows osteotomy in the apex of the deformity and because it is possible to perform lengthening and correction when necessary. We suggest performing lengthening and correction osteotomy at the same level when it is possible, whereas it is dangerous to perform it at the distal metaphyseal femur and distal third of the tibia