The aim of this study was to examine the interaction between intrinsic (player-related) and extrinsic (environment-related) variables as risk factors for anterior cruciate ligament injury in Australian football. Between 1992 and 1999, 100,820 player-match exposures were analyzed for risk of anterior cruciate ligament injury using logistic regression analysis. There were 63 surgically proven noncontact anterior cruciate ligament injuries. The strongest risk factors were a player history of anterior cruciate ligament reconstruction either in the previous 12 months (relative risk [RR], 11.33; 95% confidence interval [CI], 4.02 to 31.91) or before the previous 12 months (RR, 4.44; 95% CI, 2.46 to 8.01). Weather conditions that were associated with dry field conditions--high water evaporation in the month before the match (RR, 2.55; 95% CI, 1.44 to 4.52) and low rainfall in the year before the match (RR, 2.87; 95% CI, 1.30 to 6.32)--were also significantly associated with these injuries. The increased risk of injury in the first 12 months after reconstruction was associated with the reconstructed knee, whereas after 12 months there was an even distribution of new injuries to the reconstructed knee and contralateral knee. A history of anterior cruciate ligament reconstruction is a risk factor for further injury. Weather conditions of high evaporation and low rainfall before matches are associated with noncontact anterior cruciate ligament injury.