Plasma corticosteroid levels in 295 out-patients using halogenated topical steroids were compared with levels in 158 controls. The mean level of the treated patients was 12-5 /(g/ioo ml compared with 14 /ig, 100 ml in the controls; 96'',, of the patients who used topical steroids and 99",) of the controls had levels of 6 /fg/ioo ml or more. Five of the six patients whose plasma corticosteroid levels were less than 5 /jg/ioo ml on first sampling had normal values on retesting after an interval of 2 weeks or more.