Itching erythematous or eczematous plaques around injection sites are quite frequent side effects of heparin treatment and clinical symptoms of delayed-type non-IgE-mediated allergic hypersensitivity (DTH) to heparin. For diagnosis, intradermal, patch, and subcutaneous challenge tests with heparins are suitable. In most cases, changing the subcutaneous therapy from unfractionated to low molecular weight heparin or treatment with heparinoids does not provide improvement because of extensive cross-reactivity. Hirudin polypeptides, which exhibit a different chemical structure, are a safe therapeutic alternative for subcutaneous application, however. Importantly, despite DTH to subcutaneously injected heparins, most patients tolerate heparin intravenously. Moreover, in case of therapeutic necessity and DTH to heparins, the simple shift from subcutaneous to intravenous heparin administration without prior testing may be justified.