Twenty-nine patients with severe atopic eczema were entered into a randomized double-blind, placebo-controlled trial of the polypeptide thymus extract, thymostimulin (TP-1). The treatment period was 10 weeks with a subsequent follow-up period of up to 1 year. Of the 18 patients receiving TP-1 and the 11 patients on the placebo, 15 and 11 patients, respectively, were fully evaluable. There was no significant difference in either clinical or immunological status between the two treatment groups at baseline. At the 3-month follow-up clinic visit there was a statistically significant difference in the total clinical score, calculated as a percentage of baseline, in favour of the TP-1-treated patients. This difference was not maintained in the subsequent follow-up period and was not accompanied by an improvement in the patient's subjective well-being. There was no significant difference between the treatment groups with respect to the immunological parameters as measured at the 3-month clinic visit.