Heterotopic ossification in the abductor region of the hip following reamed intramedullary femoral nailing has an incidence as high as 68 per cent. A definitive triggering factor for heterotopic ossification remains obscure, but it has been suggested that there may be both local and systemic influences. Previous work has only been able to show a statistical correlation with systemic factors. Sixty antegrade femoral nailings were performed in 58 patients, of which 32 were unreamed. There was no significant difference between the two groups for systemic risk factors known to have statistical correlation with the formation of heterotopic bone. The incidence of heterotopic ossification in the reamed nail group was 35.7 per cent and 9.4 per cent in the unreamed nail group (P = 0.01). The difference in the incidence of heterotopic bone formation seems to be due to local factors, in particular the generation of osteogenic reaming debris, which are important in the pathophysiology of heterotopic ossification seen in femoral intramedullary nailing.