An apparatus and method for insertion of medical devices is disclosed. One embodiment includes an access tube with a retention portion disposed at a distal end. The retention portion is reconfigurable between first and second configurations of reduced and increased lateral extent. A trocar can be movably disposed within the access tube, which can be movably disposed within an overtube. One method of the invention includes inserting the trocar and access tube through a patient's body wall where the retention portion is configured from a reduced to an increased lateral extent. An overtube can be inserted around the access tube through the body wall where the retention portion can be received through a slot defined by the overtube. The retention portion can be returned to reduced lateral extent. The access tube can be removed and the medical device can be inserted within the overtube.