We studied 45 patients aged 14-66 years who had undergone stem cell transplantation for a variety of malignant conditions at least 12 months previously. Compared to normal controls, they had significantly reduced absolute numbers of CD4+, CD4+ CD45RA+ and CD4+ CD45RO+ T cells and a reduced CD4+ CD45RA+:CD4+ CD45RO+ ratio. In all subsets T-cell numbers were significantly greater 24 months, compared to 12-24 months, after transplantation and there was a nonsignificant trend towards lower T-cell numbers with increasing age. We conclude that the thymus, or putative thymic-equivalent tissue, remains functional in older adults.