Extracorporeal photochemotherapy (ECP) has been shown to be a potent activator of peripheral blood macrophages because it causes a marked release of macrophage-dependent proinflammatory cytokines, and it is therefore currently considered to be a safe and non-toxic immunomodulatory treatment. On this basis we studied the function of peripheral blood mononuclear cells (PBMC) in eight patients with early stage (Ib) cutaneous T-cell lymphoma (CTCL), before and 1 year after ECP, together with their clinical and histological responses. In particular we evaluated in vitro phytohaemagglutinin (PHA)-stimulated proliferation and production of interleukin-4 (IL-4) and interferon-&ggr; (IFN-&ggr;) as well as lipopolysaccharide (LPS)-induced production of IL-12. Before treatment we observed that PBMC of patients produced significantly higher levels of IL-4 and lower levels of IFN-&ggr; and IL-12 than those of healthy control subjects. After 1 year of ECP, IL-4, IFN-&ggr; and IL-12 production no longer differed from that of control subjects. Moreover, we observed a good clinical result matched by histological response. Our data confirm that early- stage CTCL patients show a predominantly type-2 immune response that might be responsible for several immunological abnormalities found in this disease. We have demonstrated that ECP reverses the T-helper type 1/T-helper type 2 (Th1/Th2) imbalance and may therefore be considered an efficient biological response modifier.