Diisocyanates continue to be one of the most frequent causes of occupational asthma worldwide. They are still indispensable in industrial use as components of coatings, glues, and polyurethane foams. In Germany, respiratory diseases due to diisocyanates can be compensated by the statutory accident insurance (according to BK-Nr. 1315). The present case report shows a rare case of sensitization against diisocyanates verified by skin prick test and serological testing. Due to these findings, a modified inhalation test with an extremely low initial diisocyanate concentration in the laboratory was performed, and a positive reaction could be detected already after an extremely low diisocyanate concentration. In addition, increases of fractional exhaled nitric oxide (FeNO) and eosinophils in induced sputum after inhalation testing were seen. The present case constellation underlines the particular importance of allergological tests for diagnostic clarification of the diagnosis of diisocyanate asthma.