The Bristol Helicobacter Project is an ongoing, pragmatic, double-blind placebo-controlled trial of the effect of Helicobacter pylori eradication on symptoms of dyspepsia, health utilization and costs, and quality of life in the adult population. Commencing in 1996, 27,536 individuals ages 20-59 years who were registered with seven primary care centers in Bristol and the surrounding areas in southwest England were invited to undergo a 13C urea breath test. There was no selection on the basis of symptoms and 23.5% had dyspepsia on entry to the study. A total of 10,537 people were tested (38.3% of those invited), 1636 tested positive (15.5% of those tested), and 1558 (95.2% of those who tested positive) were randomized to H. pylori eradication therapy or placebo. The rate of participation in the screening phase increased with age (odds ratio [OR]: 1.42 per decade, 95% CI: 1.31 to 1.54) and female gender (OR: 1.35, 95% CI: 1.27 to 1.43) but decreased with lower socioeconomic status (OR: 0.70, 95% CI: 0.56 to 0.86 comparing lowest with highest category). H. pylori prevalence increased with age (OR: 1.69 per decade, 95% CI: 1.51 to 1.89) and lower socioeconomic status (OR: 1.33, 95% CI: 1.05 to 1.69) but was lower in women (OR: 0.87, 95% CI: 0.76 to 1.00). Population-based trials of H. pylori eradication are feasible but necessitate screening large numbers of people to identify those who are infected and who may benefit from eradication. In the Bristol Helicobacter Project the rate of participation varied inversely with both social deprivation and the prevalence of the infection.