Abstract: Background/Objective: Hepatitis B virus (HBV) infection in patients who lack detectable surface antigen (HBsAg) is considered occult and may be found in patients with chronic HCV infection at various frequencies (50 to 87%). In Egypt, the last decade showed a remarkable decline in HBV infection associated with a rise in HCV infection. This study investigates the prevalence of occult HBV in children and adolescents with hematological diseases and in adults with chronic liver disease (CLD) in relation to HCV infection. Methods: 100 children with hematological disorders (n=50) and malignancies (n=50) (group 1; median age 11.5 yrs) and 100 CLD adult patients (30 of whom had hepatocellular carcinoma, HCC) (group 2; median age 50 yrs) were enrolled in this study. Sera were tested for HCV antibodies, HCV-RNA (nested RT-PCR), HBV markers (HBsAg, Anti-HBcAb IgM, HBeAg) and HBV-DNA (nested PCR for surface, core & x regions). Results: Anti-HCV was detected among 49% in group 1 (36 HCV-RNA+) and 92% in group 2 (47 HCV-RNA+). HBV-DNA was positive among thirty eight (38%) patients (positive c-region in 35, s-region in 4 and x-region in one) and forty one (41%) patients (positive c-region in 39, s-region in 12 and x-region in one) in groups 1 and 2 respectively. Overt HBV infection was diagnosed among 45 patients in group 1 (22 of whom were under chemotherapy) and 30 patients in group 2 (11 of whom had HCC). On the other hand, twenty one patients (21%) had occult HBV infection (9/21 had hematological malignancies) in group 1 (12 HBV-DNA+/HCV-RNA+) and thirty (30%) in group 2 (21 HBV-DNA+/HCV-RNA+; 3/21 had HCC). Occult HBV infection was significantly increased in group 1 and 2 patients with HCV infection (p=0.009 and p=0.007 respectively). Sixty one (61%) children and adolescents in group 1 had history of HBV vaccination. Conclusion: Occult HBV infection (particularly core DNA) is highly prevalent in HCV-infected patients and may clearly have clinical implications in the pathogenesis and therapy of HCV-induced chronic liver disease.