Bone tunnel enlargement has been reported after anterior cruciate ligament (ACL) reconstruction surgery. Although the long-term outcome of this phenomenon is not yet known, tunnel lysis or expansion may be clinically significant in revision surgery because the enlarged tunnels may complicate graft placement and fixation. There any many proposed theories for tunnel lysis. The most accurate statement is that this condition has a multifactorial etiology. Mechanical and biological causes have been reported, and both contribute to enlarged graft tunnels. This article describes the multiple causes of bone tunnel enlargement after ACL surgery. Future techniques and advances in primary ACL surgery must seek to eliminate this phenomenon.