Results indicated that almost 30 percent of MHOs were habitual offenders before their final homicide, yet 38 percent had no prior arrests. The majority of MHOs, however, had persistent and extensive prior criminal records that began during adulthood. Offenders who had committed rape or burglary at the time of the homicide event and those who had prior rape convictions were more likely to kill multiple victims. The findings demonstrate that a criminal career perspective can enhance the study of MHOs. Court and correctional records for 494 single homicide offenders (SHOs) and 160 MHOs were obtained from a clearinghouse with access to criminal justice information in 45 States and Washington, DC. Variables under analysis included gender, age, ethnicity, criminal onset, gang involvement, contemporaneous offenses, and prior criminal history. Data on MHOs and SHOs were compared using differences of means t tests and logistic regression models. An interesting research question to emerge from the findings involved the seeming tension between the research that has established the early childhood onset of the very worst criminal careers and the current finding that most MHOs are not arrested until adulthood. Tables, notes, references