Oro-cutaneous fistula is a serious complication following oral reconstruction. It is caused by a decline in blood coagulation factor XIII activity. We studied the relationship between the occurrence of postoperative fistula and blood factor XIII activity. Twenty-nine patients who underwent oral reconstruction at the Department of Oral Surgery, Hirosaki University from November 1991 through August 1994 were studied. Blood factor XIII activity and serum albumin concentrations were measured. Factor XIII activity decreased to the lowest level of 54.7±4.2%, as compared with baseline, on the 7 th postoperative day (POD), and gradually increased to 89.1±5.2% before the 21st POD. On the other hand, the serm albumin concentration decreased to the lowest value of 5.7±0.1g/dl on the 3 rd POD. Nine of 29 patients whose factor XIII activity decreased below 60% on the 3 rd POD had oro-cutaneous fistulas on the 5.6±0.58 POD on average. The incidence of fistula was significantly higher, in patients whose factor XIII activity was below 60% than in those whose factor XIII was above 60%. There was no correlation between the serum albumin concentration and the incidence of fistula. In the 9 patients of the 29 patients who had postoperative fistulas, mean factor XIII activity was 44.6±3.8%. These patients were given plasma factor XIII Their blood factor XIII levels increased promptly, and 7 cases of oro-cutaneous fistula were cured by 17±4.4 POD on average. In conclusion, we demonstrated that a postoperative decrease in blood factor XIII activity was an important causative factor of oro-cutaneous fistula after oral reconstruction. The results suggest that administration of plasma factor XIII is effective in preventing and treating fistulas in patients whose blood factor XIII activity is below 60% postoperatively.