Allergic contact dermatitis is a common disease whose specific therapy is avoidance of allergens. It is critical to identify frequency of occurrence of allergic reactions to initiate preventive measures (e.g. elimination of substances with an especially high allergic potential). This study analyzes results of patch testing performed between 1980 and 1993 at the University Clinic of the RWTH Aachen. Additionally, age and sex-related distribution was evaluated. The data is compared to published patch test results covering the years 1995-2001 and patch test results from 2004 in the University Clinic Aachen.