The article evaluates crime trends in south border American and Mexican sister cities using panel data analysis. The region offers a unique assessment opportunity since cities are characterized by shared cultural and historical legacies, institutional heterogeneity, and disparate crime outcomes. Higher homicide rates on the Mexican side seem to result from deficient law enforcement. Higher population densities in Mexican cities appear to also be a factor. Cultural differences, on the other hand, have been decreasing, and apparently do not play a substantial role. The homicide rate dynamics show opportunistic clustering of criminal activity in Mexican cities, while no clustering is found on the American side. Crime also appears to spill from Mexican cities into American cities. Homicide rates on both sides of the border have been falling faster than countrywide rates, leading, in the case of American cities, and against stereotypes, to rates below the countrywide rate in 2001.