Many studies have indicated a close association between gastric mucosa-associated lymphoid tissue (MALT) lymphoma and Helicobacter pylori ( Hp) infection. Follicular gastritis (FG), a rare type of gastritis, is also closely associated with Hp infection and histologically similar to MALT lymphoma. However, there are few studies investigating the relationship between FG and MALT lymphoma. To clarify the issue, we examined the B-cell monoclonality in FG by immunoglobulin heavy chain (IgH) gene rearrangement. Seventy FG patients (all Hp-positive, 23 males and 47 females, median age 33 yr) were investigated; 70 age-and gender-matched non-FG Hp-positive controls and 24 non-FG Hp-negative controls were also examined. Two gastric biopsies, one from the antrum and one from the corpus, were obtained from each patient. DNA was extracted from formalin-fixed, paraffin-embedded biopsy specimens. IgH gene rearrangement was examined by semi-nested polymerase chain reaction (PCR). In the antral mucosa, monoclonal B-cell populations were detected in 19 (32%) of 60 FG, in 6 (10%) of 60 Hp-positive controls, and in none of 20 Hp-negative controls. In the corporal mucosa, monoclonal B-cell populations were detected in 14 (30%) of 47 FG, in 6 (11%) of 54 Hp-positive controls, and in none of 20 Hp-negative controls. The incidence of monoclonal B-cell populations in the FG patients was higher than in both Hp-positive and Hp-negative controls ( P<0.05). The monoclonal B-cell populations disappeared after successful Hp eradication in 8 of 8 FG patients examined. These data suggest that FG may be strongly associated with MALT lymphoma, and that Hp eradication therapy may be indicated in FG.