Causes of Helicobacter pylori (HP)-negative gastric ulcers (GUs) have not previously been systematically studied. In this study we examined possible causes of HP-negative GUs. Among 115 consecutive patients with endoscopic diagnoses of GU, 71 (62%) had HP infection, and 44 (38%) were found to be HP-negative. Of the 71 HP-positive patients, 47 (66%) had no other detectable causal factors, 21 (30%) were regularly taking nonsteroidal antiinflammatory drugs (NSAIDs), and three (4%) had malignant GU. Of the 44 HP-negative patients, 29 (66%) were taking NSAIDs, two (5%) had a malignant GU, whereas 13/44 (30%) (11% of total 115) patients had no identifiable cause. These patients were classified as having "idiopathic GU." Some 38/115 (33%) GUs occurred on the lesser curve, and these were more often (p = 0.012) HP-positive (76%) than prepyloric GUs (60/115 (52%)). We conclude that although most patients have an identifiable cause of GU, around 11% will have no apparent contributing factor. HP-positive GUs are more frequently located on the lesser curve. It is now important to identify the cause of GU, because this may dictate choice of management.