Mixed martial arts (MMA) competitions have gained much popularity, and the sport is watched by many millions annually. Despite ongoing controversy, there have been no objective studies of the injuries sustained in MMA based on on-site evaluation. In the study reported in this article, we attempted to delineate injury patterns for MMA participants. We conducted an observational cohort study of MMA competitions held in Hawaii between 1999 and 2006. The study included 116 bouts, involving 232 "exposures" and 179 male participants between ages 18 and 40. All the fighters were examined by 1 of 4 physicians, both before and after each bout. Fighters were referred to an emergency department when necessary, and follow-up was recommended as needed. Among the 232 exposures were 55 injuries: 28 abrasions and lacerations (6 requiring on-site suturing or referral to an emergency department for suturing), 11 concussions (4 with retrograde amnesia), 5 facial injuries (2 nasal fractures, 1 tympanum rupture, 1 temporomandibular joint sprain, 1 Le Fort fracture), and 11 orthopedic injuries (3 metacarpal injuries, with 1 confirmed fracture; 1 acromioclavicular separation; 1 traumatic olecranon bursitis; 1 elbow subluxation; 1 midfoot sprain; 1 aggravation of elbow medial collateral ligament sprain; 1 elbow lateral collateral ligament strain; 1 trapezius strain; 1 Achilles tendon contusion). We describe the injuries sustained in MMA competition to make comparisons with other sports. We discuss distribution and mechanism of injuries as well as injury incidence based on on-site evaluation in MMA.