In 16 out of 18 patients with rosacea, mites of the species Demodex folliculorum were isolated in mostly great numbers from lesions of the facial skin. In patients with perioral dermatitis, the mite was demonstrated in 17 out of 29 patients. All patients, however, including those where no mites were found, could be completely cured by nightly application of hexachlorocyclohexane 0.25% (Jacutin®). The average treatment time was 7 weeks for rosacea and 5 weeks for perioral dermatitis. 2 patients with rosacea and 4 patients with perioral dermatitis relapsed, but cleared again after treatment was repeated. The initial flare-up of the dermatitis, which is observed regularly after starting the Jacutin-therapy, is described. Quantitative aspects of Demodex infestation and its histopathologi-cal alterations are discussed in relation to present-day knowledge.