Forty-nine subjects received two doses of oral cholera vaccine consisting of whole cells plus recombinant B subunit; this was followed by a booster dose one year later. After the primary series, a significant (greater than twofold) increase in the levels of vibriocidal, anti-cholera toxin immunoglobulin G and anti-cholera toxin immunoglobulin A antibodies occurred in 54, 88, and 81% of the subjects, respectively. Within one year, titers decreased to levels close to baseline. A booster dose then induced rises similar to those which occurred after the initial vaccination. The results suggest that 1-year booster doses may be necessary to maintain immunity against cholera in Latin America.