Triglycidyl isocyanurate (TGIC) is mainly used in polyester-based powder paints, but also in laminates, insulating varnishes, coatings and adhesives. Several cases of contact allergy to TGIC have been reported during the last 10 years. Contact allergy to TGIC has developed in a factory producing the chemical, in a factory producing powder paints containing TGIC, and in industries using powder coating. In this paper, we report a man who developed a work-related dermatitis when working on the painting of metal frames. He was exposed to polyester powder pigments containing TGIC. When patch tested, he was negative to TGIC (prepared in 1988) 3x and positive to polyester powder pigment. Only when a new test preparation of fresh TGIC powder was tested, was a positive reaction obtained. Chemical analyses showed that there was no TGIC in the test preparation from 1988, and that in the TGIC powder from 1988, there was only 30% of the expected amount of TGIC. The investigations, clinical and chemical, strongly indicate degradation of TGIC in the test preparation and powder. Both substances and the test preparations made from them may change over time. Therefore, if a false-negative reaction due to a test preparation is strongly suspected, we recommend a re-test of the patient with a new test preparation of fresh material. As a general rule, patch testing should be performed with fresh substances and test preparations made from them, unless their stability and durability are known.