To characterize patterns of Emergency Medical Technician (EMT) partner familiarity in three Emergency Medical Services (EMS) agencies. STUDY DESIGN/DATA SOURCES: We utilized a case study design and retrospective review of administrative data from three EMS agencies and 182 EMTs over 12 months. We used the Kruskal-Wallis test and Bonferroni corrected p-values to compare measures of partner familiarity. Measures included the annual mean number of partners, rate of partners per 10 shifts, mean shifts per EMT, and proportion of shifts worked with same partner. We standardized select measures by size of agency to account for a greater number of possible partnerships in larger agencies.Across all agencies, the mean number of shifts worked annually by EMTs was (mean [SD]) 77.3 (59.8). The unstandardized mean number of EMT partnerships was 19.3 (12.4) and did not vary across EMS agencies after standardizing by agency size (p=.328). The unstandardized mean rate of EMT partnerships for every 10 shifts worked was 4.0 (2.7) and varied across agencies after standardizing (p<.001). The mean proportion of shifts worked with the same partner was 34.8 percent and varied across agencies (p<.001).There was wide variation in select measures of EMT partner familiarity.