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The significance of reports1,2, that topical application of house dust mite (HDM) to abraded or excoriated uninvolved skin of patients with atopic eczema can provoke an eczema-like reaction is controversial. We have therefore studied 17 adult in-patients with atopic eczema who demonstrated positive prick tests to Bencard HDM solution. One ml of Bencard HDM solution was applied, under a crepe bandage, daily for 5 days to an untreated 10 cm2 mildly eczematous area behind one knee, and diluent only, similarly, to the other knee in a double-blind fashion. Test sites were assessed daily for itch using a visual analogue scale and for severity of erythema, papules and excoriation using a 3-point grading system. Comparing these parameters between active and control sites for each patient, local worsening of eczema in response to HDM was marked in 5/17, moderate in 3/17 and mild in 4/17 subjects, while mild deterioration with diluent only occurred in 3/17. Mean area of active eczema increased by 6 26 ± 8 4 cm2 (± SD) at HDM-treated and by 0 5 ±1.5 cm2 at control sites (P≤0.01; paired t-test). Further patients exposed to Bencard horse hair extract, to which they were prick-test negative, in place of HDM, showed no reaction. These results indicate that HDM can exacerbate pre-existing atopic eczema.