CVD 103-HgR is a live oral cholera vaccine strain constructed by deleting 94% of the gene for the enzymatically active A subunit of cholera toxin from classical Inaba Vibrio cholerae O1 569B; the strain also contains a mercury resistance gene as an identifying marker. This vaccine was well tolerated and immunogenic in double-blind, controlled studies and was protective in open-label studies of volunteers challenged with V. cholerae O1. A randomized, double-blind, placebo-controlled, multicenter study of vaccine efficacy was designed to test longer-term protection of CVD 103-HgR against moderate and severe El Tor cholera in U.S. volunteers. A total of 85 volunteers (50 at the University of Maryland and 35 at Children's Hospital Medical Center/University of Cincinnati) were recruited for vaccination and challenge with wild-type V. cholerae El Tor Inaba. Volunteers were randomized in a double-blind manner to receive, with buffer, a single oral dose of either CVD 103-HgR (2 x 108 to 8 x 108 CFU) or placebo (killed E. coli K- 12). About 3 months after immunization, 51 of these volunteers were orally challenged with 105 CFU of virulent V. cholerae O1 El Tor Inaba strain N16961, prepared from a standardized frozen inoculum. Ninety-one percent of the vaccinees had a μ4-fold rise in serum vibriocidal antibodies after vaccination. After challenge, 9 (39%) of the 23 placebo recipients and 1 (4%) of the 28 vaccinees had moderate or severe diarrhea (μ3-liter diarrheal stool) (P < 0.01; protective efficacy, 91%). A total of 21 (91%) of 23 placebo recipients and 5 (18%) of 28 vaccinees had any diarrhea (P < 0.001; protective efficacy, 80%). Peak stool V. cholerae excretion among placebo recipients was 1.1 x 107 CFU/g and among vaccinees was 4.9 x 102 CFU/g (P < 0.001). This vaccine could therefore be a safe and effective tool to prevent cholera in travelers.