A standardized skin surface biopsy was performed in 34 patients suffering from skin diseases with high Demodex folliculorum density (Dd) > 5D/cm2 before, during and after topical treatment. The patients were randomized into six comparable groups to study six topical treatments: metronidazole 2%, permethrin 1%, sublimed sulphur 10%, lindane 1%, crotamiton 10% and benzyl benzoate (BB) 10%. Their acaricidal activity was measured according to three criteria: (i) for each treatment, decrease of Dd to under the normal threshold (< or = 5 D/cm2); (ii) for each treatment, a significant decrease in Dd; and (iii) comparison of the relative difference in Dd between treatments. These three criteria converged to establish the acaricidal activity of BB on D. folliculorum; the efficacy of crotamiton was demonstrated by the second criterion. An important irritating effect was observed with BB and sulphur.