Background
Hydroxyisohexyl 3-cyclohexene carboxaldehyde (HICC) has been the most reported fragrance chemical for two decades and will be prohibited in cosmetic products from August 2021.

Objectives
To describe the time trend of HICC contact allergy in European patients with dermatitis in 2009 to 2019, and the added value of testing HICC separately in the baseline series.

Methods
Data were reviewed for 124 472 patients with dermatitis who were patch tested with HICC 5% pet. in the baseline series in the European Surveillance System on Contact Allergy (ESSCA) network (2009 to 2018) and at the Herlev-Gentofte Hospital Department of Dermatology and Allergy (2009 to 2019).

Results
Contact allergy to HICC was found in 1.98% of 9865 patients in Gentofte and 1.62% of 114 607 patients in the ESSCA network. Overall, the prevalence decreased annually, with 0.156 percentage points (P = .001) in Gentofte and 0.051 percentage points (P = .0002) in ESSCA. The frequency of missed contact allergy to HICC when testing only with fragrance mix II (FMII) was 0.17% (17/9865) and 0.35% (405/114607) of the whole test population in the Gentofte and ESSCA populations, respectively.

Conclusions
This is the first study to demonstrate a significant decline in HICC allergy in European patients with dermatitis, most likely attributed to the upcoming European ban.