The study shows that during this period, Puerto Rico had lower annual rates and faster improvement in aggravated assault, rape, and robbery than the mainland; higher homicide rates than the mainland and no sign of converging to mainland rates; and a higher lethality ratio from aggravated assault than the mainland. Puerto Rico's average annual homicide rate during 1999-2005 was 19 victims per 100,000 residents, almost 4 times higher than the rate in the States of Florida or New York (5 victims/100,000 residents) and more than twice as high as the rate in Mississippi (9 victims/100,000 residents). The comparative, descriptive analysis of homicide rates using selected metropolitan areas and States produced essentially the same result as the analysis of homicide rates that compared the average rate for all of Puerto Rico to the average rate for the entire mainland. The author suggests that future research must find ways to obtain improved data on these and other variables that could be helpful in explaining different patterns of violent crime and homicide in Puerto Rico compared with the mainland. The author discusses possible explanations for two of these findings: Why the lethality of aggravated assault in Puerto Rico surpassed the lethality of aggravated assault on the mainland; and why--despite many years of persistent poverty, unemployment, and socioeconomic and political marginality--Puerto Rico has experienced less violent crime, other than homicide, than the mainland. Some of the factors examined pertained to the number of police and quality of policing, emergency response time, medical advances, community characteristics that determine crime reporting to police, and the existence of gated poor communities in Puerto Rico. 3 figures, 3 tables, 6 notes, and 29 references