To assess hepatitis C virus (HCV) incidence rates and identify determinants of infection among hemodialysis patients, a multicenter study was conducted in 58 units in Italy. An initial seroprevalence survey was conducted among 3,492 patients already on hemodialysis therapy as of January 1997 and among an additional 434 patients who began dialysis up to January 1998. HCV antibodies were assessed by third-generation enzyme Immunoassays. Patients testing seronegative at baseline were enrolled Into a 1-year incidence study with serological follow-up at 6 and 12 months. For patients who seroconverted, an HCV RNA assay was performed on stored baseline samples to confirm new infection. A nested case-control study was subsequently performed to Investigate potential risk factors. For each incident case, three controls negative for both HCV antibodies and HCV RNA were randomly selected. At enrollment, HCV seroprevalence was 30.0%. During follow-up, 23 new HCV cases were documented, with a cumulative incidence of 9.5 cases/1,000 patient-years. By logistic regression analysis, an increased risk for HCV infection emerged for patients attending the dialysis units with a high prevalence of HCV-infected patients at baseline (odds ratio OR, 4.6) and for those attending units with a low personnel-patient ratio (OR, 5.4). Among extradialysis factors, a history of surgical Intervention in the previous 6 months (OR, 16.7) significantly Increased HCV risk. These findings suggest that the combination of understaffing and a high level of Infected patients in the dialysis setting increases the risk for HCV nosocomial transmission. This is likely related to an increased likelihood for breaks in infection control measures. Supported in part by the Progetto Epatite Vrale-ISS(D.L.vo 30/12/1992 n. 502), and Ricerca Corrente-IRCCS Spallanzani.