Post-marketing Surveillance Study of Ex-intravenous Drug Abusers With Chronic Hepatitis C Treated With PegIntron Plus Rebetol (P04408/MK-4031-261)

Inclusion Criteria: * Treatment-naïve participants or relapsers to interferon monotherapy * Participants with chronic hepatitis C infection * At least 18 years of age * Must meet the following laboratory criteria: * Platelets \>=100,000/mm\^3 * Neutrophil count \>=1,500/mm\^3 * TSH (thyroid stimulating hormone) within normal limits * Hemoglobin \>=12 g/dL (females); \>=13 g/dL (males) * Ex-intravenous drug abusers who are under stable substitution therapy * Women of childbearing potential must practice adequate contraception and have a routine pregnancy test performed monthly during treatment and for 7 months post-treatment. * Sexually-active participants must be practicing acceptable methods of contraception during the treatment and for 7 months post-treatment Exclusion Criteria: * Any contraindications specified in the SPC (Summary of Product Characteristics) and approved European labeling * Hypersensitivity to the active substance or to any interferons or to any of the excipients * Pregnant women * Women who are breast-feeding * Existence of or history of severe psychiatric condition, in particular severe depression, suicidal ideation or suicide attempt * A history of severe pre-existing cardiac disease, including unstable or uncontrolled cardiac disease in the previous 6 months * Severe debilitating medical conditions, including participants with chronic renal failure or creatinine clearance \<50 mL/min * Coinfection with HIV (Human Immunodeficiency Virus) * Autoimmune hepatitis or history of autoimmune disease * Severe hepatic dysfunction or decompensated cirrhosis of the liver * Pre-existing thyroid disease unless it can be controlled with conventional therapy * Epilepsy and/or compromised central nervous system function