There's a lot of research out there; here's a few academic studies cited by The Harvard School of Public Health with their conclusions.
(http://www.hsph.harvard.edu/research/hicrc/firearms-research/guns-and-death/index.html)
Hepburn, Lisa; Hemenway, David. Firearm availability and homicide: A review of the literature.Aggression and Violent Behavior: A Review Journal. 2004; 9:417-40.
Case-control studies, ecological time-series and cross-sectional studies indicate that in homes, cities, states and regions in the US, where there are more guns, both men and women are at higher risk for homicide, particularly firearm homicide.
Hemenway, David; Miller, Matthew. Firearm availability and homicide rates across 26 high income countries. Journal of Trauma. 2000; 49:985-88.
We found that across developed countries, where guns are more available, there are more homicides.
Miller, Matthew; Azrael, Deborah; Hemenway, David. Household firearm ownership levels and homicide rates across U.S. regions and states, 1988-1997. American Journal of Public Health. 2002: 92:1988-1993.
After controlling for poverty and urbanization, for every age group, people in states with many guns have elevated rates of homicide, particularly firearm homicide.
Miller, Matthew; Azrael, Deborah; Hemenway, David. State-level homicide victimization rates in the U.S. in relation to survey measures of household firearm ownership, 2001-2003. Social Science and Medicine. 2007; 64:656-64.
2001-2003. We found that states with higher levels of household gun ownership had higher rates of firearm homicide and overall homicide.
And one from the economist Richard Florida http://www.theatlantic.com/national/archive/2011/01/the-geography-of-gun-deaths/69354/
States with tighter gun control laws appear to have fewer gun-related deaths.