The term systemic contact dermatitis is used to describe a dermatosis seen in some persons with delayedtype hypersensitivity to a hapten administered systemically. Immunohistochemical studies of flare-up reactions seen after oral challenge indicate that the mechanism includes delayed-type hypersensitivity, but clinical observations suggest that other mechanisms may also be involved. The clinical spectrum includes flares of previous sites of dermatitis, flexural dermatitis, the baboon syndrome, eruptive vesicular hand eczema, and a nonspecific maculopapular rash. General symptoms such as headache, malaise, and arthralgia occur occasionally. The reactivation of previously positive patch test sites is an important, apparently immunologically specific reaction seen only in experimental oral challenge studies. Systemic contact dermatitis caused by uncommon haptens (such as certain drugs) confirm the existence of this entity. Reactions to ubiquitous haptens (such as the metals nickel, cobalt, and chromium and common food flavorings) are more difficult to identify with certainty because it is difficult to control exposure to these haptens. Thus, the existence of clinically relevant cases of systemic contact dermatitis caused by these substances has been questioned. Delayed-type hypersensitivity to these ubiquitous haptens is so common that research in this area has important implications for many patients with allergic contact dermatitis.