The prognostic significance of six urinary modified nucleosides, 5-methylcytidine (5-MeCyd), 4-acetylcytidine (4-AcCyd), 1-methylinosine (1-MeIno), 1-methyladenosine (1-MeAdo), 7-methylguanosine (7-MeGua) and pseudouridine (psi-Urd) was evaluated in 68 breast cancer patients of the specialized cancer hospital of Lyon (France). Excretions of 1-MeIno and 1-MeAdo were significantly higher in patients hospitalized in the medical rather than surgical ward, reflecting more advanced disease, and also among patients who died within 5 years of follow-up as compared to those still alive. These results suggest an unfavourable prognostic significance of high urinary excretion of 1-MeIno and 1-MeAdo in breast cancer patients.