AIMS--To investigate the effects of longer term corticosteroid treatment on circulating lymphocyte subsets. METHODS--Prednisolone (20 mg daily) was given to 12 healthy volunteers in a single morning dose for three days. Circulating lymphocyte subsets were measured by flow cytometry after whole blood lysis. RESULTS--Seven hours after the first dose of prednisolone there was a significant fall in absolute numbers of lymphocytes, T cells, CD4+ and CD8+ cells, and B cells. The percentage of T cells fell significantly, due to a fall in percentage of CD4+ cells. In contrast to the seven hour findings, at 72 hours there was a significant rise in absolute numbers of lymphocytes, T cells, CD4+, CD8+, and B cells. This trend was already apparent by 24 hours. The percentage of CD4+ cells was significantly raised at 72 hours, while that of CD8+ cells had fallen significantly. The percentage of natural killer cells had fallen at 72 hours; that of B cells remained increased at 72 hours. CONCLUSIONS--These findings show that corticosteroid treatment causes significant changes in lymphocyte subsets, and that such changes must be considered when designing studies of lymphocyte subsets during illness.