This study seeks to analyze the trend of homicide rates (total and by firearm) in Mexico between 1990 and 2009 and identify the variables that best explain the geographical variations of these rates in the 2008-2009 two-year period. Homicide rates, adjusted for age, were calculated for both sexes between 1990 and 2009 and for each state in 2008-2009. Factors associated with the interstate variations in the homicide rates were identified using multiple linear regression analysis. Results show that the homicide rate in Mexico decreased between 1990 and 2007, but doubled over the last two years (from 7.6 to 16.6 per 100,000). In 2009, the male homicide rate was almost 9 times higher than the female rate and about two-thirds of homicides involved firearms. Multivariate analysis reveals that impunity, drug trafficking, alcohol and drug consumption and school dropout in basic education in that order - are key factors for understanding the geographical variations in homicide rates in Mexico in 2008-2009. Findings suggest that to reduce the number of homicide victims and spatial variations in the rate, it is necessary not only to fight the drug cartels, but above all to implement structural reforms in the criminal justice system and reduce the socioeconomic disparities among states.