Thirty-nine patients with tibial plateau fractures and concomitant ligament injury were evaluated at least one year after injury. Ligamentous injury was determined by stress roentgenograms, plain roentgenograms, operative findings, and Pelle-grini-Stieda's ossification. There were 22 isolated medial collateral, eight lateral collateral, one isolated anterior cruciate, and eight combined ligament injuries. All types of tibial plateau fractures were associated with ligamentous injury, although split compression and local compression were most common. Twenty patients (Group 1) did not have operative repair of the injured ligaments, and 19 patients (Group 2) had primary repair of the injured ligaments. Open reduction and internal fixation of the plateau fracture(s) were performed in 13 patients in Group 1 and 19 patients in Group 2. Follow-up evaluation (100-point scale), including subjective, functional, and anatomic factors, revealed 12 excellent and good, four fair, and three poor results in the 19 patients with ligamentous repair. There were ten excellent and good, two fair, and eight poor results in those without ligament repair. Ten of the 12 patients with 10 degrees or more of instability had poor results. These poor results included five unrepaired medial collateral ligaments, two unrepaired lateral collateral ligaments, and three patients with cruciate ligament injury. This study confirms the view that instability is a major cause of unacceptable results in tibial plateau fractures. Operative repair of medial and lateral collateral ligaments, with appropriate treatment of the bony plateau fracture, may reduce late instability and may improve overall morbidity in these concomitant injuries. Cruciate ligament injury associated with a tibial plateau fracture carries a poor prognosis.