This article examines the relationship between firearm availability and homicide rates in Detroit, Michigan. Noting the difficulties involved in measuring gun density, the analysis uses an indicator based on the relative frequency with which firearms are employed in robberies and suicides. Models estimated from time-series data are consistent with the argument that higher levels of firearm density increased the risk of homicide in the city. A variety of supplementary analyses support this finding and suggest that the effect of gun availability on Detroit’s homicide rates is relatively large.