Background: Axillary dermatitis is common and overrepresented in individuals with contact allergy to fragrances. Many individuals suspect their deodorants to be the incriminating products. Objective: Our aim was to investigate the significance of cinnamic aldehyde in deodorants for the development of axillary dermatitis when used by individuals with and without contact allergy to cinnamic aldehyde. Methods: Patch tests with deodorants and ethanol solutions with cinnamic aldehyde, and repeated open application tests with roll-on deodorants without and with cinnamic aldehyde at different concentrations, were performed in 37 patients with dermatitis, 20 without and 17 with contact allergy to cinnamic aldehyde. Results: A repeated open application test with positive findings was noted only in patients hypersensitive to cinnamic aldehyde (P < .001) and only in the axilla to which the deodorants containing cinnamic aldehyde had been applied (P < .001). Conclusion: Deodorants containing cinnamic aldehyde in the concentration range 0.01% to 0.32%, used twice daily on healthy skin, can elicit axillary dermatitis within a few weeks. (J Am Acad Dermatol 2003;48:194-200.)