Hepatitis C virus (HCV) RNA and genotypes, as well as markers of hepatitis B virus infection, were surveyed in 171 patients with chronic liver disease, 276 patients with chronic renal failure, and 961 blood donors in Thailand. HCV RNA was detected in 30 (23%) of 128 patients with nonalcoholic chronic liver disease and hepatitis B surface antigen (HBsAg) in 60 (47%), and both HCV RNA and HBsAg in 3; the cause of liver disease was not established in 41 (32%) patients. HCV RNA was detected in 44 (20%) of 221 patients on maintenance hemodialysis or with kidney transplantation, but in none of 55 patients on peritoneal dialysis. Antibodies to synthetic HCV core peptides were detected in 39 (4.1%) of sera from 961 blood donors, and HCV RNA was detected in 8 (0.8%). Of the 90 HCV RNA samples from patients and donors, genotype V prevailed (46%) followed by II (22%), I (14%), III (3%), and VI (2%); genotypes were not classifiable into any of I-VI in the remaining 10%. There were six sera which contained HCV RNA, but were without antibody to HCV detectable by the second-generation enzyme immunoassay. HCV RNA titers were high in four patients with kidney transplantation, but low in one patient with chronic liver disease and one patient on maintenance hemodialysis. HCV RNA at high titer (≥104/ml) was not classifiable in one patient. These results indicate HCV of novel genotypes in Thailand, seronegative HCV infection in patients with kidney transplantation, and a low risk of HCV infection in patients treated by peritoneal dialysis. © 1994 wiiey-Liss, Inc.