PURPOSE: To evaluate the results of open tibial fractures treated with unreamed AO intramedullary nail MATERIALS AND METHODS: Among the patiens of open tibial fractures who were treated with unreamed AO intramedullary nail from January 1993 to August 1998, the authors reviewed 34 patients whose follow-up was possible for more than one year. Fourteen patients were treated with AO external fixator after meticulous debridement, followed by unreamed AO intramedullary nailing, and the other 20 patients were treated with primary unreamed AO intramedullary nailing after debridement. The authors evaluated the results by the union time and the presence of complication. RESULTS: The average union time was 26.3 weeks. There was no significant difference of union time and complications between the patients who were treated with AO exteranl fixator followed by unreamed AO intramedullary nailing and the patients who were treated primarily with debridement and unreamed AO intramedullary nailing. However there was high incidence(21%) of interlocking screw breakage. CONCLUSION: With meticulous debridement and close observation of the wound, the open tibial fractures can be effectively treated with unreamed AO intramedullay nailing. Unreamed intramedullary nailing of open tibial fractures immediately after trauma can be recommended in selected cases.