Perioral dermatitis (POD) is a common dermatosis and is considered by most dermatologists to be increasing in incidence. An Australia-wide, questionnaire-based study investigating the aetiology of POD was conducted, with particular emphasis on the significance of cosmetic use. One hundred and thirty-three cases were obtained from dermatologists across Australia and were compared with 99 randomly selected controls who were matched for age and sex. Application of foundation, in addition to moisturizer and night cream resulted in a 13-fold increased risk for POD (odds ratio 13.5; P < 0.001). The combination of moisturizer and foundation was associated with a lesser but significantly increased risk for POD (odds ratio 2.9; P = 0.017). Moisturizer alone was not associated with an increased risk of POD. These findings suggest that cosmetic preparations play a vital role in the aetiology of POD, perhaps by an occlusive mechanism. In 83% of cases, topical steroid application to the face occurred after the development of a facial rash.