Firearm availability is measured by an index composed of two indicators: proportion of Detroit robberies committed with a gun, and proportion of Detroit suicides committed with a gun. The level of gun density is viewed as both a cause and a consequence of the homicide rate. Use of the gun density index assumes that as firearms become more available, they will be employed more frequently by robbers and by persons wanting to commit suicide. The study results are consistent with the argument that gun density influences homicides. Changes in firearm availability alter the risk of homicide, and the results suggest that Detroit's homicide rate would be appreciably lower if gun density in the city could be reduced. Because there appears to be nothing unique about Detroit's criminal, social, or economic environment that would influence the association between firearm availability and homicide rates, the findings may be generalizable to other cities. 43 references, 23 notes, and 3 tables (Author abstract modified)