Eighteen osteoligamentous elbow joint specimens were included in a study of the medial collateral ligament complex (MCL). The morphologic characteristics of the MCL were examined, and three-dimensional kinematic measurements were taken after selective ligament dissections were performed. On morphologic evaluation the MCL is divided into the anterior bundle and the posterior bundle. The anterior bundle can be divided into anterior and posterior bands. The maximum valgus and internal rotatory instability after transection of the anterior band, 11.7 degrees and 11.2 degrees, respectively, were found at elbow flexions of 30 degrees and 40 degrees. Severance of the entire anterior bundle produced major valgus and internal rotatory laxity through the complete flexion arc of maximal 14.2 degrees and 18.5 degrees, respectively, at 70 degrees and 60 degrees of elbow flexion. Cutting both the posterior band and the posterior bundle resulted in only internal rotatory laxity of maximal 7.2 degrees at 130 degrees of elbow flexion. This study defines the anterior band as the primary constraint to valgus and internal rotatory forces, the posterior band as the secondary, and the posterior bundle as the tertiary constraint. The MCL was observed to be a complex of ligamentous fibers rather than individual bands that stabilizes the joint against valgus and internal rotatory forces.