In view of the wide variety of components currently used in metalworking fluids (MWF), relevant contact sensitizations may be overlooked, because commercially available MWF test series cannot cover the full spectrum. Hence, patch testing with MWF from the patient’s workplace is an important additional diagnostic tool. However, recommendations on how to perform such patch tests vary. We retrospectively analyzed patch test data of the Department of Dermatology in Dortmund, 1992-2003. In 141 metalworkers tested because of suspected occupational contact dermatitis due to MWF, 829 patch tests with 306 samples of MWF were performed. Water-based MWF (wb MWF) were mainly tested in 2 dilution series, i.e. pure (workplace concentration), 10%aq. and 1%aq., and pure, 50%aq. and 10%aq. Positive reactions to wb MWF occurred in 27 patients. Patch testing with wb MWF at workplace concentration resulted in 16.1 %(39/242) positive reactions, with a positivity ratio of 69%and a reaction index of 0. From the analysis of reaction patterns and concomitant reactions,we conclude that most of these positive reactions indicated true contact allergy. With lower concentrations, relevant allergic reactions may be missed. Neat oils were tested as is or diluted from 1 to 50%in olive oil, but no reactions at all were observed. For optimum benefit of patch testing with MWF from the patient’s workplace, breakdown testing is recommended. To overcome the time-consuming difficulties associated with this procedure, we propose a centre for information and documentation of contact allergies due to occupational exposure. Furthermore, full declaration of MWF ingredients is desirable. In view of the wide variety of components currently used in metalworking fluids (MWF), relevant contact sensitizations may be overlooked, because commercially available MWF test series can not cover the full spectrum. Therefore, patch testing with MWF from the patient’s workplace is an important additional Diagnostic tools. However, recommendations on how to perform such patch tests vary. We retrospectively analyzed patch test data of the Department of Dermatology in Dortmund, 1992-2003. In 141 metalworkers tested because of missing occupant contact dermatitis due to MWF, 829 patch tests water-based MWF (wb MWF) were mainly tested in 2 dilution series, ie pure (workplace concentration), 10% aq. and 1% aq., and pure, 50% aq. and 10 % aq. Positive reactions to wb MWF occurred in 27 patients. Patch testing with wb MWF at workplace concentration resulted in 16.1% (39/242) positive reactions, with a positivity ratio of 69% and a reaction index of 0. Fro m the analysis of reaction patterns and concomitant reactions, we conclude that most of these positive reactions indicated true contact allergy. With lower concentrations, relevant allergic reactions may be missed. Neat oils were tested as is or diluted from 1 to 50% in olive oil For optimum benefit of patch testing with MWF from the patient’s workplace, breakdown testing is recommended. To overcome the time-consuming difficulties associated with this procedure, we propose a center for information and documentation of contact allergies due to occupational exposure. Furthermore, full declaration of MWF ingredients is desirable.