Sera from 50 patients with culture-proven campylobacter gastroenteritis were examined for the presence of antibodies to Legionella pneumophila. Ten patients (20%) had a positive titre (> or = 16) as measured by indirect immunofluorescence. Antibodies were detected in only 1 of 36 acute sera but in 10 of 14 (71%) sera obtained more than 10 days after the onset of symptoms. All positive sera contained specific IgM antibodies but specific IgG or IgA could not be detected in any sample. No legionella antibodies could be detected in sera from 42 similar patients with salmonella gastroenteritis. These results were shown to be due to serological cross-reaction between L. pneumophila and campylobacter.