Increased anticardiolipin antibodies (aCL) serum levels have been recently described in haemodialysis patients and in renal transplant recipients, with a prevalence ranging from 4.8 to 46.4%. The causes and the clinical significance of aCL positivity in these patients are uncertain. We measured IgG- and IgM-aCL serum levels in 61 haemodialysis patients, in 14 renal transplant recipients and in 38 healthy controls. Increased levels of IgG-aCL were found in 4 haemodialysis patients (6.55%), in 2 transplant patients (14%) and in 2 of the healthy controls (5.26%). IgM-aCL serum levels were normal in all the patients. After one year of follow-up, no vascular events have been observed in aCL positive patients. It is probable that the presence of aCL in the serum of patients with end-stage renal disease is only an epiphenomenon and does not play a pathogenetic role.