Campath‐1H, an anti‐CD52 monoclonal antibody, is therapeutically active in lymphoproliferative and autoimmune diseases. After Campath‐1H therapy, lymphocytes with a paroxysmal nocturnal haemoglobinuria (PNH) phenotype have been reported to emerge. We characterized a PNH‐like lymphocyte population emerging after Campath‐1H therapy, in a patient with fludarabine refractory B‐cell chronic lymphocytic leukaemia (B‐CLL). We demonstrated a reduction in PIG‐A mRNA levels compared with controls, and of all cytokines tested [interleukin (IL)‐4, IL‐13, IL‐2, interferon(IFN)‐γ, IL‐6, IL‐10, and tumour necrosis factor (TNF)‐α], except transforming growth factor (TGF)‐β. Given the inhibitory activity of TGF‐β, its elevated levels may contribute to the selective pressure of Campath‐1H, leading to the emergence of PNH‐like lymphocytes.