To assess the effect on knee motion of gradual femoral lengthening using an intramedullary nail, between 1994 and 2003, 27 non-achondroplastic patients had bilateral femoral lengthening using the Albizzia nail. Vigorous post-operative physiotherapy was the norm. Knee motion recorded at various stages pre and post-operatively was compared. For an average gain of 6 cm the mean flexion during lengthening was 119. By final follow-up all patients had regained pre-operative range of motion. No tenotomies or joint manipulations were required. Good knee motion can be maintained during femoral lengthening using an intramedullary lengthening device.