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Hepatitis C virus (HCV) prevalence is high among injection drug users (IDUs) in Afghanistan. Duration of injection and young age at first injection are common risk factors for HCV in IDU populations. The association of HCV with these time factors was analyzed.Socio-demographic and drug use behavior information were collected. Participants had rapid testing for HCV with recombinant immunoblot assay confirmation. Modeling of non-linear associations was performed using fractional polynomial logistic regression.Among 459 male IDUs, age at first injection had a constant HCV risk (odds ratio (OR): 1.01 per year; 95% confidence interval (CI): 0.98–1.03), while each additional year of injection drug use had a significantly increased risk (OR: 4.72 per year, 95% CI: 2.92–7.66). HCV risk increased significantly with each additional year of injecting drug use by groups of injectors: young (≤22 years, OR: 1.97; 95% CI: 1.27–3.07), middle (23–28 years, OR: 1.76; 95% CI: 1.28–2.43) and older (≥29 years, OR: 7.56; 95% CI: 3.15–18.14).The probability of HCV infection increased markedly by duration of injection drug use and varied according to age at first injection. Drug counseling and educational efforts should be directed to older drug users who have not yet initiated injecting and to young IDUs to avert infection and reduce risky drug use behaviors.