The September 11, 2001 terrorist attacks permanently transformed aviation security, generating more intensive security-related practices. Although these enhanced security measures primarily sought to prevent future terrorist attacks, they also may have provided a secondary benefit of reducing property crimes at airports. The present case study examines changes in airport security at Chicago's O'Hare International Airport post-9/11 in the context of routine activities theory. The study first posits that increased security measures at O'Hare should have resulted in increased capable guardianship, thereby reducing the number of suitable targets and criminal opportunities for motivated offenders. After identifying various changes in airport security at O'Hare since 9/11 in the context of routine activities theory, the study then examines changes in larcenies from January 1999 to June 2006 using an interrupted time series analysis. The ARIMA model indicates that in the post 9/11 O'Hare Airport environment, there was a statistically significant and persistent reduction in reported larcenies.