The prevalence and incidence of hepatitis C virus (HCV) infections were studied in 236 dialysis patients and related to clinical data at two hospitals in Stockholm, Sweden. Patients were followed during 12 months and tested by 1st- and 2nd-generation anti-HCV assays. Time of seroconversion to HCV could be determined by retrospective analysis of stored serum samples. A total of 36 (15%) patients were anti-HCV positive. Time of seroconversion could be determined for 23 patients and was in the majority of cases associated with blood transfusions, but late seroconversion (more than 6 months after transfusion) as well as lack of transfusion in some cases implied that HCV might be transmitted through dialysis equipment. Persistence of elevated alanine amino-transferase levels for more than 6 months occurred in 17% of anti-HCV-positive patients. In conclusion, routes of transmission in dialysis units have to be further evaluated since routes other than transfusion may occur and diagnosis may be delayed in this group of patients probably due to a poor immunological response.