If a positive patch test reaction occurs in the absence of sensitization to a certain antigen, we call it a false positive reaction. There may be several reasons for this: impurities of the test substances, too high a concentration, wrong choice of test location and reading time; positive reactions against test vehicle, or adhesive tape, nonspecific reactions due to adjacent strongly positive patch test reactions and finally wrong interpretation by the doctor. Diagnosis of contact allergy, especially to potent allergens or topical medications and questions relating to occupational disease may be of eminent importance to the patient. Therefore false positive as well as false negative test reactions should be recognized; this also appears particularly important for medicolegal reasons.