This article focuses on the effect of urbanization on violent crime – particularly homicide in Costa Rica. Although violence is a major problem throughout Latin America, few empirical studies carried out in the area use high-quality socioeconomic and crime databases with a high level of geographical disaggregation. In this article, we employ data from all 473 districts of Costa Rica between 2010 and 2013. We develop a model which takes into account endogeneity problems and uses contrasts of marginal linear predictions. We conclude that the degree of urban concentration plays a key role in explaining homicide rates, other things being equal. This effect is progressive: the greater the urban concentration, the greater the increase in homicide rates. This causal relationship is not observed in offenses other than homicide.