Objective: To carry out an anatomical study of the medial collateral ligament, an important elbow stabilizer in different degrees of elbow flexion-extension. Methods: Forty elbows were dissected in order to analyze the functional behavior of the anterior, posterior and transverse ligament bands during valgus stress maneuvers of the elbow in different degrees of flexion and extension. Two groups were determined; in the group GPA the posterior band of the ligament was sectioned initially, then the articular capsule and finally the anterior band; in group GAP this order was reversed. Results: Instability was observed in GPA only in the third stage, when there was a greater mean elbow's opening during the flexion (between 50° and 70°); in GAP, the instability was present since the first stage; the degrees of flexion with greater instability were the same as in group GPA. Conclusion: The anterior band of the medial collateral ligament of the elbow is the most important stabilizer of the elbow valgus instability, and its principal action occurs between 50° and 70° of elbow flexion. Level of Evidence III, Diagnostic Studies - Investigating a diagnostic test.206334338Shiba, R., Sorbie, C., Siu, D.W., Bryant, J.T., Cooke, T.D., Wevers, H.W., Geometry of the humeroulnar joint (1988) J Orthop Res., 6, pp. 897-906Wadström, J., Kinast, C., Pfeiffer, K., Anatomical variations of the semilunar notch in elbow dislocations (1986) Arch Orthop Trauma Surg., 105, pp. 313-315Davidson, P.A., Pink, M., Perry, J., Jobe, F.W., Functional anatomy of the flexor pronator muscle group in relation to the medial collateral ligament of the elbow (1995) Am J Sports Med., 23, pp. 245-250Morrey, B.F., An, K.N., Stability of the elbow: osseous constraints (2005) J Shoulder Elbow Surg., 14 (SUPPL 1), pp. 174-178Knutzen, K.M., Hamill, J., (2008) Bases biomecânicas do movimento humano, , São Paulo: Manole, 2 edRichard, M.J., Aldridge, J.M., Wiesler, E.R., Ruch, D.S., Traumatic valgus instability of the elbow: pathoanatomy and results of direct repair. Surgical technique (2009) J Bone Joint Surg Am., 91 (SUPPL 2), pp. 191-199Morrey, B.F., An, K.N., Functional anatomy of the ligaments of the elbow (1985) ClinOrthop Relat Res., 201, pp. 84-90Callaway, G.H., Field, L.D., Deng, X.H., Torzilli, P.A., O'brien, S.J., Altchek, D.W., Biomechanical evaluation of the medial collateral ligament of the elbow (1997) J Bone Joint Surg Am., 79, pp. 1223-1231Lech, O., Pitágoras, T., Barbosa, J., Severo, A., Reconstrução cirúrgica das lesões ligamentares agudas do cotovelo (1998) Rev Bras Ortop., 33, pp. 945-950Field, L.D., Callaway, G.H., O'brien, S.J., Altchek, D.W., Arthroscopic assessment of the medial collateral ligament complex of the elbow (1995) Am J Sports Med., 23, pp. 396-400Timmerman, L.A., Andrews, J.R., Histology and arthroscopic anatomy of the ulnar collateral ligament of the elbow (1994) Am J Sports Med., 22, pp. 667-673Bennett, J.B., Green, M.S., Tullos, H.S., Surgical management of chronic medial elbow instability (1992) Clin Orthop Relat Res., 278, pp. 62-68Schwab, G.H., Bennett, J.B., Woods, G.W., Tullos, H.S., Biomechanics of elbow instability: the role of the medial collateral ligament (1980) Clin Orthop Relat Res., 146, pp. 42-52Søjbjerg, J.O., Ovesen, J., Nielsen, S., Experimental elbow instability after transection of the medial collateral ligament (1987) Clin Orthop Relat Res., 218, pp. 186-190Conway, J.E., Jobe, F.W., Glousman, R.E., Pink, M., Medial instability of the elbow in throwing athletes (1992) Treatment by repair or reconstruction of the ulnar collateral ligament. J Bone Joint Surg Am., 74, pp. 67-83Pollock, J.W., Brownhill, J., Ferreira, L.M., McDonald, C.P., Johnson, J.A., King, G.J., Effect of the posterior bundle of the medial collateral ligament on elbow stability (2009) J Hand Surg Am., 34, pp. 116-123Vieira, E.A., Caetano, E.B., Bases anátomo-funcionais da articulação do cotovelo: contribuição ao estudo das estruturas estabilizadoras dos compartimentos medial e lateral (1999) Rev Bras Ortop., 34, pp. 481-488Pichora, J.E., Fraser, G.S., Ferreira, L.F., Brownhill, J.R., Johnson, J.A., King, G.J., The effect of medial collateral ligament repair tension on elbow joint kinematics and stability (2007) J Hand Surg Am., 32, pp. 1210-1217Hotchkiss, R.N., Weiland, A.J., Valgus stability of the elbow (1987) J Orthop Res., 5 (3), pp. 372-377Cage, D.J., Abrams, R.A., Callahan, J.J., Botte, M.J., Soft tissue attachments of the ulnar coronoid process (1995) An anatomic study with radiographic correlation. Clin Orthop Relat Res., 320, pp. 154-158Morrey, B.F., Complex instability of the elbow (1998) Instr Course Lect., 47, pp. 157-164Motta Filho, G.R., Malta, M.C., Lesões ligamentares agudas do cotovelo (2002) Rev Bras Ortop., 37, pp. 369-380Morrey, B.F., Master techniques in orthopaedic surgery (1994) The Elbow, , New York: Raven Press