Dermatitis may occur as a result of exposure to chemicals in the workplace. Approximately 30 percent of cases of chemical dermatitis are classified as allergic contact dermatitis (ACD). Irritant contact dermatitis (ICD) is due to the irritant effect of chemicals and is much more common. Toluene diisocyanate (TDI) is one of many chemicals, which are both weak skin sensitizers and irritants. Diphenylmethane diisocyanate (MDI) has also been reported to be a skin sensitizer, but there is a question whether the skin reaction is due to a breakdown product, diaminodiphenylmethane (MDA), on the surface of the skin or to a cross reaction due to structural similarities. Biochemical data lends support to the latter rather than the former hypothesis. Patch testing is widely used to establish a causal relationship between ACD and a specific causative agent, and to differentiate between ACD and ICD, which often is not possible on clinical or even histological grounds. However, there are only a handful of chemicals available in standardized commercial trays in concentrations recommended by expert international organizations. Isocyanates are among the substances for which no generally accepted concentration has been adopted. Various concentrations have been recommended and are being used. As a consequence, there are real limitations in the interpretation of patch testing, particularly in the absence of expertise with the test methodology.