Because few reports have explored possible associations between viral factors and breast cancer prognosis, we investigated the presence or absence of DNA for six viruses in breast cancer patients and analyzed associations with survival rate.Polymerase chain reaction (PCR) and Southern hybridization were used to detect DNA of six viruses---human papillomavirus (HPV), cytomegalovirus (CMV), Epstein-Barr virus (EBV), herpes simplex virus (HSV)-1, HSV-2, and human herpesvirus (HHV)-8---in 69 breast cancer specimens and 44 benign specimens (12 normal samples, 32 fibroadenomas) that were used as internal controls.Five cancer specimens were negative for β-globulin and two cancer specimens represented patients with positive family histories; they were excluded from analysis.Survival of groups positive for DNA of different viruses was calculated according to Kaplan-Meier methods,and relations between virus-positive groups and survival were compared with the log-rank test.Of 62 cancer specimens, 8 (12.9%) were HSV-1 positive, but all were HSV-2 negative.Among viral DNA-positive breast cancer samples, 12 samples (16.0%) were positive for one virus, 16 (25.8%) were positive for two viruses, 21(33.8%) were for three viruses, 3 (4.9%) were positive for four viruses, and 10 (16.0%) were virus-DNA negative.Comparing survival of different groups as defined by positive status for virus DNA showed significantly worse survival for women with breast cancer whose tumor was positive for one virus (p=0.0005).Interaction of multiple viral factors during development and progression of breast cancer may alter cell tropisms and neutralize viral factors that are aggressive when present singly.