A series of 169 consecutive patients from low socioeconomic strata attending the gastroenterology clinic of Charity Hospital in New Orleans were evaluated clinically and endoscopically. This general New Orleans population is known to be at increased risk of developing gastric carcinoma. The type of gastritis was identified histologically, and the presence of Campylobacter pylori was determined by culture and/or histology. The overall prevalence of C. pylori infection in this patient population was 71% (126/169). These findings were correlated with serum IgG antibody to C. pylori using an ELISA. Fifteen patients with neither demonstrable gastritis nor C. pylori served as negative controls and had low levels of IgG antibody to C. pylori. A strong correlation was found between Campylobacter detection by morphologic and/or culture technique and the presence of serum IgG antibody. For all patients examined, the sensitivity of the ELISA was 94.2% and the specificity 75.5%. The highest ELISA values for IgG antibody (sensitivity = 89%, specificity = 75%) were detected in patients positive for C. pylori, who also had diffuse antral gastritis with prominent lymphoid follicles. For patients with chronic atrophic gastritis and intestinal metaplasia, the sensitivity of the ELISA was 96% and the specificity 67%. The latter number may indicate underrepresentation of foveolar epithelium in biopsies with extreme intestinal metaplasia. Results suggest a high prevalence of chronic infection of C. pylori in this clinic population. The possible role of C. pylori in the development of precursor lesions of gastric cancer is discussed.