Of the three cities examined in this paper, Chicago was unusual, in that its murder books were kept by the Chicago Police Department. The city grew tenfold between 1870 and 1930 (300,000 to 3 million), a rate exceeded only by Los Angeles. The five decades between 1870 and 1930 saw the United States become an urban Nation, with its population reaching 50 percent urban by 1920. Between 1850 and 1870, the nature of gun ownership probably changed. Although it may well have been higher in the colonial period, the mass manufacturing era introduced inexpensive concealable weapons. Murders in both New York City and Los Angeles reflected the change; in both cities, guns accounted for about 7 percent of murders before 1851 and approximately 25 percent (Los Angeles) and 22 percent (New York City) from 1855 through 1875. In comparing age standardized rates of homicide for Chicago and New York City from 1880 through 1930 (the two largest U.S. cities at the time), several observations are important. First, the long series for New York City shows several surprises: a relatively high rate throughout two centuries, a low period in 1900 when one would have predicted a high period, and a high rate at the beginning of the 19th century, in all probability much higher than the current rate. Because the data are reported on the decade, at least two important high peaks for New York are invisible, those for 1857-1858 and 1862-1864, both of which exceeded rates at the end of the 20th century. Second, the 1930 peak for Chicago masks a downturn, because the city's crude rate had reached a high point in 1925 and then began to decline. Chicago was a city that confirmed the theory that urban growth leads to high rates of violence. During the period, Chicago doubled New York's homicide rate. Apparently local and regional factors, rather than national demographic trends, hold the key to explaining homicide rates in the three cities during the period studied. 1 table and 2 figures