During five football seasons, from 1989 through 1993, 61 surgically proven, noncontact, anterior cruciate ligament injuries, were identified from among 22 National Football League teams. The variables of surface, shoe type, playing conditions, and whether or not the shoe was spatted were identified for each reported injury. Forty noncontact injuries occurred in conventional cleated shoes on natural grass, and 21 occurred on an artificial surface. Almost half of all injuries (47.5%) occurred during game-day exposures despite the fact that the practice versus game-day exposure rate was 5:1. Of these injuries, 95.2% (N = 58) occurred on a dry field. The factors of cleat and shoe type, type of surface (natural versus artificial), surface conditions (wet/dry), and the effect of "spatting" a shoe are presented. The significance of these factors and their likelihood to be associated with injury is analyzed by use of a statistical tool, the incidence density ratio.