Background: Opium is the most commonly abused drug in Iran. However, effects of opium abuse or withdrawal on vascular endothelium are not clear. This study compared endothelial function in addicts, non-addicts, and individuals receiving treatment for addiction. Methods: This descriptive, analytic study was conducted in Isfahan (Iran) during 2011. Nitric oxide (NO) and C-reactive protein (CRP) levels were evaluated in four groups of individuals (addicts, non-addicts, persons receiving methadone maintenance treatment, and those attending drug rehabilitation camps). Findings: The mean serum levels of high sensitivity CRP (hs-CRP) were 3.59 ± 1.20 in non-addicts, 4.10 ± 1.25 in addicts, 3.29 ± 0.97 ng/ml in subjects receiving methadone, and 5.33 ± 1.21 in residents of camps (P < 0.05). The mean serum levels of NO in the abovementioned groups were 31.29 ± 4.68, 22.51 ± 4.53, 25.51 ± 4.22, and 20.03 ± 3.56 ng/ml, respectively (P < 0.01). Conclusion: Among the four studied groups, opium addicts had the highest levels of CRP and the lowest levels of NO. Therefore, opium results in vascular dysfunction which can be corrected during the withdrawal process. Keywords: Opium, Addiction, Endothelial function