Erythematous, infiltrated plaques appear to be a common but neglected cutaneous reaction to heparin. Erythematous, infiltrated plaques are unrelated to heparin necrosis and sometimes closely mimic contact dermatitis. We report 15 patients (14 women and 1 man, the first to be reported in the literature) in whom erythematous, infiltrated plaques developed 3 to 21 days after commencement of subcutaneous heparin therapy. The clinical appearance, routine histopathologic and immunohistopathologic findings, and results of various skin tests provided circumstantial evidence for the presence of a delayed hypersensitivity reaction. Subcutaneous provocation tests proved superior to intracutaneous or epicutaneous tests for the diagnosis of erythematous, infiltrated plaques. Erythematous, infiltrated plaques were caused by heparin constituents in all female patients, whereas chlorocresol was implicated as the cause in the only man.