Norwalk virtls infection was sought in 48 US, 49 Puerto Rican, and 27 Mexican adults attending medical school in Guadalajara (Mexico) who were enrolled in a 2-year longitudinal study. Serum specimens were collected quarterly and as acute- and convalescent-phase samples around episodes ofgastroenteritis. The reciprocal Norwalk virus geometric mean titer (GMT) for Puerto Rican students (567) was significantly higher than that of the US students overall (294; P<.001) and for four of nine quarterly periods. The reciprocal Norwalk GMT for Mexican students (748) was also significantly higher than that of the US students overall (P<.001) and for seven of nine quarterly periods. The average percentage of students per year with seroconversions was 30%. The rate of Norwalk virus infection averaged 0.36 episodes per student-year. Symptoms of gastroenteritis associated with seroconversion occurred in 45% of students. Preexisting serum antibody did not protect against subsequent Norwalk virus infection in these subjects. All student groups had similar rates of infection and symptomatic gastroenteritis.