Of 275 patients with eczema and tested routinely with phenylacetaldehyde, 4 reacted positively. In addition, 3 became sensitized by testing. Cross-sensitivity to closely related substances could not be demonstrated. Gaschromatography showed that the substance used was pure. Since phenylacetaldehyde appears to be a potent sensitizer, it must be questioned whether it should be used in the manufacture of perfumes and flavouring agents.