Three hundred and fifty-four patients with symmetrical dermatoses took part in a multicentre, doubleblind, half-side study in order to compare the efficacy of a new topical steroid, diflucortolone valerate 0.3% (Nerisone Forte) against that of an established, potent topical steroid, clobetasol propionate 0.05% (Dermovate). The assessment of overall response, as judged by the physicians' preference for one side or another, showed no difference between the two compounds. However, when the results were examined by separate diagnostic category, the number of preferences was greater for diflucortolone valerate 0.3% in eczema, and for clobetasol propionate 0.05% in psoriasis, although neither of these differences reached levels of statistical significance. The graded assessments of response indicated that both compounds were highly effective, potent, topical steroids. Eighty-one percent of all patients showed marked improvement or healing with diflucortolone valerate 0.3%, and 84% showed marked improvement or healing with clobetasol propionate 0.05%. This difference was not statistically significant. Analysis of response, either by diagnosis or grade of severity, showed no statistically significant differences between the two compounds. No significant differences in the incidence of severity of side-effects were observed. It was concluded that the two compounds were of equal clinical efficacy.