Helicobacter pylori has been suggested as a cause of gastric carcinoma and gastric non-Hodgkin's lymphoma (NHL). In a previous cohort study, a relative risk of six for gastric NHL was reported among subjects who tested positive for anti-H.pylori antibodies. The association between peptic ulcer and NHL has been studied in a population-based case-control investigation on hemato-lymphopoietic malignancies in Italy, based on face-to-face interviews to 2671 cases and 1718 controls (refusal rates 10 and 19%, respectively). Subjects who reported a diagnosis of peptic ulcer had a relative risk of 5.6 [95% confidence interval (CI) 3.8-8.0] for gastric NHL, whereas the estimate for non-gastric NHL was 1.3 (1.0-1.6). The association with recent diagnosis of ulcer was stronger, but the odds ratio (OR) was as high as 2.1 (95% CI 1.1-4.2) after >/=20 years since such diagnosis. After exclusion of the last 2 years before the diagnosis of NHL, and of ulcers diagnosed before 1978 (when gastroscopy became common in Italy), the OR was still 5.3 (95% CI 3.0-9.2). We found a strong effect modification by educational level, with ORs for ulcer more elevated in higher social groups. Gender was an effect modifier (OR = 4.1 in males, 9.2 in females; P = 0.03 for heterogeneity). The association with other gastrointestinal pathologies was much lower and statistically not significant. Almost all gastric lymphomas were B-cell NHLs of intermediate grade according to the working formulation; the majority belonged to the mucosa-associated lymphoid tissue (MALT) type. The association with ulcer was much stronger among MALT lymphomas, but only for recent ulcer diagnoses (2-10 years). Our study shows an increased risk for gastric NHL, very similar to the estimate reported in a previous cohort study. The risk was higher among more educated subjects.