Two outbreaks of varicella consisting of a total of 105 cases occurred in a highly varicella-susceptible population of young adult Army recruits from Puerto Rico enrolled in the Defense Language Institute in San Antonio, Tex, between October 1986 and November 1987. Epidemiologic investigation found a significantly higher risk for enlisted recruits housed in open barracks than for officers housed in private rooms. The attack rate in the first outbreak was 30%, with an estimated attack rate of 71% among susceptible persons. Serologic testing of 810 adult recruits from Puerto Rico for varicella-zoster antibody by means of an enzyme-linked immunosorbent assay procedure found 42% to be seronegative, with no significant difference by sex. The enzyme-linked immunosorbent assay test had a positive predictive value for absence of disease development of 95% in the second outbreak. Serologic test results were successfully used as part of the outbreak control strategy, with a resultant decrease in attack rates to 19% overall and 30% among susceptible persons in the second outbreak. Uniquely susceptible adult populations placed in conditions with high likelihood of infection on exposure are potential candidates for the varicella vaccine after its licensure.