Hepatitis C virus (HCV) prevalence among high-risk pediatric and adult patients was evaluated. The study included 269 adults and 150 children in a case-control research design. Risk factors of HCV exposure in Indonesia were assessed among adult renal dialysis patients and pediatric patients who received multiple blood transfusions. A high prevalence of anti-HCV was found among the adult renal dialysis patients, measured by second-generation electroimmunoassay tests. Family members of dialysis patients, who served as a comparison group for dialysis patients, were found to have a 9.0% seroprevalence. The prevalence of anti-HCV among pediatric patients with hematological disorders was found to be 39.0%. The comparison group seroprevalence (pediatric patients and family members) was 4.3% among sera available for confirmatory testing. Patients with history of hospitalization (odds ratio [OR] = 7.94, 95% confidence interval [CI]: 4.06-15.51, P = 0.0001), blood transfusion (OR = 6.85, 95% CI: 3.95-11.88, P = 0.0001), circumcision (OR = 2.39, 95% CI: 1.43-3.99, P = 0.0001), or marital partner/family member history of jaundice (OR = 3.62, 95% CI: 1.97-6.62, P = 0.0001) were found to have an increased odds of HCV exposure compared with individuals without similar histories.