The usefulness of a single Widal test to diagnose typhoid fever in endemic areas was investigated. Reciprocal Salmonella typhi O and H titers ⩾40 and ⩾80, respectively, occurred in approximately 90% of 42 Mexican patients with bacteriologically-confirmed typhoid fever at the time of presentation to hospital and, by day 4 to 5 of clinical illness, in 70% of U.S. adult volunteers who developed typhoid fever in the course of vaccine efficacy trials but in only 0.7% (O) to 3% (H) of 275 healthy individuals from a non-endemic area. Healthy Peruvians from areas endemic for typhoid fever commonly had antibody which was age-related. Peak prevalence was found in 15- to 19-yr-olds in whom 29% had O titers ⩾40 and 76% had H titers ⩾80. A single Widal test in an unvaccinated individual showing elevated O and H titers is strongly suggestive of typhoid fever if the person comes from a non-endemic area or is a child less than 10 yr of age in an endemic area. Because of the high prevalence of antibody amongst healthy individuals over 10 yr of age in endemic areas, a single Widal test offers virtually no diagnostic assistance in adolescents and adults.