Background: Since discovered in 1990, Cag A, a protein expressed by specific strains of Helicobacter pylori, was thought able to explain why only a few Helicobacter infected patients develop peptic diseases and gastric cancer. However, clinical trials provide discordant results. Materials and Methods: In this study we evaluate Helicobacter pylori and Cag A seropositivity in 35 cancer affected patients, in 36 gastritis affected patients and in 40 healthy blood donors by means of two comercially available fluorescence enzyme-immunoessay (ELISA). Results: Odds ratios determination strongly suggests that Cag A bearer Helicobacter strains play a pathogenetic role in gastric diseases (OR 4.23, 95% CI 3.22-5.24 for cancer versus healthy volounteers, OR 3.2, 95% CI 2.19-4.21 for gastritis versus asymptomatic patients), but is unable to demonstrate a direct carcinogenic activity (cancer-gastritis difference is not significant: OR 1.32, 95% CI 0.39-1.25). Conclusions: Cag A seropositivity can be considered a risk factor for peptic disease, and only indirectly for gastric carcinoma. The paper also discuss some sampling, laboratory and statistical bias that can explain a wide eterogenity of the results reported in the literature.