Heterotopic ossification (HO) of the patellar tendon following intramedullary nailing of the tibia for fracture, which has not been previously documented, occurred in 2 of 31 patients reviewed who had adequate follow-up after this procedure. One suffered grade IV HO and the other grade II HO using a modification of the Brooker classification. The patient with grade IV HO had serious disability from pain and decreased range of motion. Risk factors previously suggested were associated with the formation of heterotopic bone in our two patients. These factors included head injury and long-term hypoxemia.