AbstractBackgroundPolyaminopropyl biguanide (INCI name) and polyhexamethylene biguanide (PHMB) are polymeric biguanides. PHMB is a broad‐spectrum antimicrobial substance used as a preservative in many products. Due to our limited knowledge on PHMB contact allergy frequency and the fact that cases of allergic contact dermatitis to PHMB might be missed, we have included PHMB as a screening allergen since 2016.ObjectiveTo report the prevalence of positive patch test reactions to PHMB as a screening allergen in patients with suspected allergic contact dermatitis.MethodsA retrospective analysis of 1760 patch tested patients from July 2016 to December 2018 was performed. Polyaminopropyl biguanide 2.0% aqua was included in the extended Malmö baseline series during the study period.ResultsOf all patients, 1204 (68.4%) were female. Positive patch test reactions were reported in 19 patients (1.1%). The most common sites of lesions were face, head, and neck (52.6%). There was a significant correlation between concomitant reactions to PHMB and other cosmetic‐related allergens.ConclusionThe prevalence of positive reactions to PHMB was higher than that previously reported. Patch testing with PHMB should be performed in patients with dermatitis who have lesions on the face, head, and neck.