To estimate the prevalence of hepatitis C virus (HCV) infection in dialysis patients, serum anti-HCV antibodies were evaluated in 489 Japanese patients undergoing hemodialysis, and 152 members of the hospital dialysis staff by enzyme-linked immunosorbent assays for anti-C100, anti-KCL-163 (HCV nonstructural protein), and anti-JCC (translation product of the presumptive HCV core gene). Of the 489 hemodialysis patients, 100 (20.4%) were positive for anti-C100, 107 (21.9%) for anti-KCL-163, and 168 cases (34.4%) for anti-JCC. These rates were significantly higher than those for either the hospital staff or the healthy blood donors. Forty-two per cent of the dialysis patients were anti-HCV positive by at least one assay, suggesting that HCV infection is more common among this population than previously thought. Positivity for anti-HCV was related to the duration of hemodialysis. Elevated alanine aminotransferase levels were present in 12.5% of the dialysis patients, 77% of whom were also anti-HCV positive. The positivity rates among the 152 members of the hospital staff were 0.7% for anti-C100, 2.6% for anti-KCL-163, and 8.6% for anti-JCC, with the anti-JCC rate of positivity exceeding that of the healthy blood donors.