ZAP-70 (zeta-chain-associated protein 70 kDa) expression is associated with poor prognosis in patients with chronic lymphocytic leukaemia (CLL). This study evaluated the efficacy of non-myeloablative allogeneic stem cell transplantation in patients with advanced CLL and assessed the impact of ZAP-70 expression on the outcome. Thirty-nine sequential patients were included. All had previously been treated with fludarabine. All patients received a preparative regimen of fludarabine (30 mg/m(2)/d for 3 d), intravenous cyclophosphamide (750 mg/m(2)/d for 3 d), and high-dose rituximab. Immunohistochemical techniques on marrow biopsy samples were used to determine that ZAP-70 was expressed in 25 patients, whereas 13 other patients were ZAP-70 negative, and one was of indeterminate status. With a median follow-up time of 27 months, the estimated overall survival and current progression-free survival (CPFS) rates at 4 years were 48% and 44% respectively. Patients who were ZAP-70 positive had 56% survival, and their CPFS rate increased from 30% to 53% after a donor lymphocyte infusion. Multivariate analysis indicated that chemorefractory disease and mixed T cell chimerism at day 90, but not ZAP-70 positivity, were associated with the risk of disease progression after transplantation. These results demonstrate a potent graft-versus-leukaemia effect that can overcome the adverse prognostic effect of ZAP-70 expression.