The effect of extracorporeal photopheresis (EP) on various cytogenetic parameters has been investigated. During EP the photoactivatable agent 8-methoxypsoralen (8-MOP) was administered orally. After 2 h a leukocyte-enriched blood fraction was collected by haemocentrifugation, irradiated with UVA extracorporeally, and reinfused to the patient. Two patients suffering from cutaneous T-cell lymphoma showed a marked clinical improvement in response to therapy. In order to investigate the cytogenetic effects and mutagenic risk of EP, the mitotic index (MI), the type and number of chromosomal aberrations and the rate of sister chromatid exchanges (SCE) were studied. Following EP treatment the patients' lymphocytes were cultured and stimulated with phytohaemagglutinin (PHA) for 48 or 72 h. The cultured lymphocytes showed a decreased MI after 48 h as an indicator of cytotoxic effects, but not after 72 h. In lymphocyte cultures not stimulated with PHA, the MI was decreased even after 72 h. The number of chromosomal aberrations and SCE were increased upon treatment, but only transiently, returning to basal levels between consecutive treatments. Our data provides no evidence for increased mutagenic risk as a consequence of effective EP treatment.