A proper ontological treatment of intolerance--including hypersensitivity--to various substances is critical to patient care and research. However, existing methods and standards for documenting these conditions have flaws that inhibit these goals, especially translational research that bridges the two activities. In response, I outline a realist approach to the ontology of substance intolerance, including hypersensitivity conditions. I defend a view of these conditions as a subtype of disease. Specifically, a substance intolerance is a disease whose pathological process(es) are realized upon exposure to a quantity of substance of a particular type, and this quantity would normally not cause the realization of the pathological process(es). To develop this theory, it was necessary to build pieces of a theory of pathological processes. Overall, however, the framework of the Ontology for General Medical Science (which uses Basic Formal Ontology as its uppermost level) was a more-than-adequate foundation on which to build the theory.