Nitric oxide (NO) is a free radical of physiological significance. The changes in the production of NO during the fracture healing process are not well known. This clinical prospective study was planned to determine these changes in patients with fractured long bone(s), who underwent surgery in the 3rd week after fracture. The patients were divided into two groups: 20 patients with an isolated femoral fracture and 20 patients with multiple fractures, including a femoral fracture. Venous blood was drawn from the healthy volunteers (n = 20) once, and from the patients seven times during 21 days after fracture. NO level was measured as nitrite in each serum sample. The serum NO levels at all measurements of the patients were significantly higher than those of the control subjects. The time-dependent changes in the NO levels were similar in both patient groups. The serum NO levels were highest in the first 6 hrs, and then decreased to the lowest level on the 3rd day. Subsequently, there was an increasing trend on the 5th, 7th and 14th days. At all times of the measurements, the NO levels were higher in patients with multiple fractures than in those with the femoral fracture, with the significant difference observed at the 6 hrs and on the 7th day. The NO levels were decreased on the 21st day after surgery. The present study suggests that NO production may be increased in considerable amounts during the first 2 weeks of fracture healing, particularly in the first 6 hrs.