The usefulness of a single Widal test to diagnose typhoid fever in Ethiopia was investigated in three study groups both retrospectively and prospectively. These were blood culture proven typhoid and non-typhoid patients and healthy individuals. Salmonella typhi H and O titres greater than or equal to 1:160 occurred in respectively 82% and 58% of typhoid fever patients; only 4% of healthy individuals and 8% of non-typhoid patients had Widal titres greater than or equal to 1:80. In typhoid fever, the H titre is elevated earlier and more frequently than the O titre. Antibody rise is maximal during the second week of illness. Antibiotic treatment did not affect the rise of antibody titre in typhoid fever. A single Widal test in an unvaccinated Ethiopian patient showing H and/or O titres greater than or equal to 1:160 and typhoid-like symptoms is strongly suggestive of typhoid fever. It also appears that H titre is more useful than O titre. More false positives are found than false negatives. Under these circumstances, the clinical picture is the decisive factor in making a diagnosis of typhoid.