In addition, within a given city, estimates of prevalence will be comparable over time only if the definitions used remain constant. The analysis considered definitions used in Chicago and Los Angeles. The gang cases analyzed were all designated by the gang units of each police agency. The analysis involved 135 and 148 nongang homicides from the Los Angeles Police Department and 226 gang and 200 nongang cases from the Los Angeles Sheriff's Department. The results showed that a motive-based definition of gang-related homicides yields about half as many gang homicides as does a member-based definition. Results indicated the need for caution in making comparisons across cities and time periods; similarities across places and time should also be recognized. Tables and notes.