We report an elderly patient with paroxysmal nocturnal haemoglobinuria (PNH), having recurrent enterocolitis and haemolytic attacks associated with cellular immunodeficiency. On admission, the patient had normal neutrophil count and function but a decreased T-cell count, decreased mitogenic reactions, and a negative tuberculin test. Granulocyte colony-stimulating factor (G-CSF) was administered, resulting in an increased T-cell count, normalization of T-cell function, increased blood levels of helper T cell (Th)1 and Th2 cytokines and improvement in the enterocolitis and haemolytic attacks. This suggests that G-CSF may be useful in the treatment of elderly PNH patients with cellular immunodeficiency.