The efficacy of the live oral typhoid fever vaccine Ty 21a marketed in Switzerland in 1981 was investigated. Forty-seven Swiss travellers to Third World countries contracted typhoid fever in 1982 and 1983; 20 (43%) of these persons had been vaccinated with Ty 21a. The incidence of typhoid fever in vaccinees was 1.1 per 10,000 doses of Ty 21a vaccine sold, which was similar to the incidence of 0.9 per 10,000 doses for the ineffective killed Ty 2 vaccine. Comparison of the incidence of vaccination in patients with typhoid fever in 1983 and in healthy travellers to India revealed vaccination efficacy rates which were not significantly different from zero. It is suggested that Ty 21a vaccine in the form marketed in Switzerland in 1981-1983 is not sufficiently potent or heat-stable, so that the doses of viable bacteria ingested were too low.