Since the ingestion studies by Marshall and Morris, Helicobacter pylori has been known to cause both acute and chronic infection in the human stomach activating both the cellular and the humoral immune system. It is of little or no value to evaluate the causative relationship of an infectious agent using Koch's criteria. The more recent criteria for causative relationships used in the science of epidemiology are more useful. These criteria include: (i) the characteristic of the association which is fulfilled for most cases of both duodenal and gastric ulcer; (ii) the temporal relationship which is fulfilled for duodenal ulcer and has not been investigated for gastric ulcer; (iii) the biological gradient which has been fulfilled for duodenal ulcer in a few studies but not investigated for gastric ulcer; (iv) the biological plausibility which is easily fulfilled for both duodenal and gastric ulcer; (v) the effect of an intervention which has been fulfilled for duodenal ulcer and in a few studies for gastric ulcer; and (vi) the coherence of these data with what is known about the disease which is fulfilled for both duodenal and gastric ulcer. Even though there is no need for all criteria to be fulfilled, further studies are necessary to confirm the temporal relationship between H. pylori and peptic ulcer, and the biological gradient of H. pylori in relation to the gastric ulcer. Even so, there is a strong indication that most of the peptic ulcers, apart from those caused by non-steroid anti-inflammatory drugs and Zollinger-Ellison-like syndromes, are caused by H. pylori infection.