ImportanceResident-to-resident aggression in assisted living facilities can result in physical and psychological harm, but its prevalence is unknown.ObjectiveTo estimate the prevalence of resident-to-resident aggression, including physical, verbal, and sexual, among residents in assisted living facilities.Design, Setting, and ParticipantsThis study used cross-sectional, observational data from a clinical trial, in which residents of assisted living facilities were monitored for events over a 1-month period. All residents of 14 large facilities randomly selected from 2 geographic locations (N = 1067), except those receiving hospice care (n = 11), were invited to participate; 93 died or moved prior to enrollment. There were 33 family and resident refusals; 930 residents were enrolled. Data were collected between May 30, 2018, and August 11, 2022.Main Outcomes and MeasuresThe data are from a clinical trial testing the effectiveness of an intervention to reduce resident-to-resident aggression. In addition, the study was designed to assess prevalence using the Time 1 (baseline) data, using a probability sample of facilities to allow for this analysis. Resident-to-resident aggression was identified using a mixed-method, case-finding strategy involving 6 sources: (1) cognitively capable resident reports regarding 22 possible events, (2) direct care staff report, (3) staff member reports collected from event-reporting forms, (4) research assistant observation of events in real time, (5) facility accident or incident reports, and (6) resident records.ResultsThe prevalence of resident-to-resident aggression among the 930 participants (mean [SD] age, 88.0 [7.2] years; 738 women [79.4%]) during the past month was estimated to be 15.2% (141 of 930 residents; 95% CI, 12.1%-18.8%). The most common forms of aggression included verbal (11.2% [104 of 930 residents; 95% CI, 8.8%-14.2%]), physical (41 of 930 residents; 4.4% [95% CI, 3.1%-6.3%]), sexual (0.8% [7 of 930 residents; 95% CI, 0.4%-1.6%]), and other (70 of 930 residents; 7.5% [95% CI, 5.5%-10.2%]). These categories are not mutually exclusive as residents could be involved with more than 1 type of aggressive behavior.Conclusions and RelevanceIn this cross-sectional, observational prevalence study, resident-to-resident aggression in assisted living facilities was highly prevalent. Verbal aggression was the most common form, and physical aggression also occurred frequently. The effects of resident-to-resident aggression can be both morbid and mortal; therefore, intervention research is needed to prevent it and to treat it when it occurs.