PURPOSE: To establish the incidence, type and significance of knee instability in patients with ipsilateral femoral and tibial shaft fracture, comparing with the patients with femoral shaft or tibial shaft fracture alone. MATERIALS AND METHODS: Two hundreds and seventy-nine consecutive patients were retrospectively reviewed from February 2000 to April 2004. They were composed of 80 patients with femoral shaft fracture alone, 176 patients with tibial shaft fracture alone and 23 patient with ipsilateral femoral and tibial shaft fracture. We evaluate the instability of knee based on physical examinations, plain stress films and MRI. We analyze incidence and period to diagnosis of instability, period to complete bony union and Hospital for Special Surgery (HSS) knee score respectively. RESULTS: There were 6.3% of knee instability in femoral shaft fracture alone, 9.7% in tibial shaft fracture alone and 30.4% in ipsilateral femoral and tibial shaft fracture. The average period to diagnosis of instability, average period to complete bony union and average HSS knee score were 9.2 months, 4.7 months and 65 points in femoral shaft fracture alone, 9.1 months, 4.2 months and 69 points in tibial shaft fracture alone, 8.7 months, 5.3 months (femur), 4.7 months (tibia) and 57 points in ipsilateral femoral and tibial shaft fracture respectively. CONCLUSION: We should consider MRI to evaluate the knee instability in patient with ipsilateral femoral and tibial shaft fracture at the time of injury and make a plan early about the treatment of knee instability.