The past several decades have seen a dramatic decline in the percentage of homicides cleared by arrest. The decline in clearance rates has led to an increasingly high percentage of cases where the relationship between the victim and offender is unknown. Using Qualitative Comparative Analysis (QCA) and data from the United States Supplementary Homicide Reports (SHR) for 1976 through 1998, this study examined the nature of unknown relationship homicides and changes in their structure over time. Three basic issues were explored: (1) the extent the characteristics of these homicides resembled homicides with known victim/offender relationships and the degree they were qualitatively distinct; (2) the change, if any, in the nature of homicides with unknown victim/offender relationships over time; and (3) to what degree a homicide where the victim/offender relationship was unknown could be assumed to involve strangers. The results indicate that although some cases involving unknown victim/offender relationships are quantitatively distinct, approximately half of all homicide cases fall within configurations that are common to known and unknown victim/offender relationships. In addition, strong evidence of change was seen in the structure of unknown victim/offender relationship homicides over time. The nature of the shift appears to be due to an increase in African-American victims and the use of guns. Tables, notes, references