Background and Aim Helicobacter pylori cagA can be classified into mainly two types (East-Asian-type and Western-type cagA) according to the repeat regions located in the 3' region. Recent studies showed that the Western-type cagA in strains from Okinawa, Japan formed a different cluster (J-Western-type cagA subtype). It has also been reported that J-Western-type cagA possesses a 12-bp insertion located in the 5' region of cagA sequence. Methods The prevalence of 12-bp insertion in cagA in Okinawa and the United States (U.S.) was examined by DNA sequencing. The primer pair that can detect the 12-bp insertion only by polymerase chain reaction was then designed. The prevalence of strains with 12-bp insertion was examined in 336 strains isolated from Okinawa by polymerase chain reaction. Results In case of Western-type cagA/vacA s1m2 strains, the prevalence of 12-bp insertion was significantly higher in strains isolated from Okinawa than that from the U.S. (P?=?0.002). Phylogenetic tree showed that strains with 12-bp insertion formed two individual clusters within J-Western-type cagA subtype; one is from Okinawa and another is from the U.S. The designed primer set showed high sensitivity (100%) and specificity (90.8%) in Okinawa. The 12-bp insertion was found in 23.7%, 14.3%, 4.2%, and 4.0% of strains with duodenal ulcer (DU), gastritis, gastric cancer, and gastric ulcer (GU), respectively (P?<?0.001 for DU vs GU) in Okinawa. Conclusions Although the mechanisms are unknown, the presence of 12-bp insertion was associated with the presence of DU and might have a suppressive action on GU and gastric cancer.