OBJECTIVE To report cross-ethnic differences among white and Mexican-American DUI offenders on the characteristics of an arrest (index) that brought subjects to treatment. METHOD Subjects are 250 (223 male) whites and 249 (187 male) Mexican Americans consecutively admitted to five DUI offender programs in a Northern California county. Interviews averaged 1 hour in length and were conducted in the programs by trained interviewers. The response rate was 71% for whites and 84% for Mexican Americans. RESULTS At the time of the arrest, Mexican Americans were driving cars older than those driven by whites. A higher proportion of Mexican Americans than whites reported having consumed 10 or more drinks before the arrest. The most frequent locale for drinking before the arrest was "home or friends' home" for Mexican Americans and a "bar/tavern/club" for whites. CONCLUSIONS Cross-ethnic differences related to the circumstance of the arrest (car year) do not fully explain the higher rates of DUI arrest among Mexican Americans, compared with whites. Differences in drinking locale before arrest suggest that server intervention efforts may not be as effective in preventing DUI for Mexican Americans as they are for whites. Prevention interventions with Mexican Americans should center on the family, as a means to minimize drinking at home and decrease the large number of drinks consumed on a particular occasion by Mexican Americans.