Recent epidemiologic studies have revealed that comorbidity of psychiatric disorders is far more pervasive than previously suspected. Strong associations have been reported between specific substance use disorders and between any mental disorder and any substance use disorder. This report focuses on comorbidity of nicotine dependence, a substance use disorder on which little epidemiologic information is available. Data come from an epidemiologic study of approximately 1000 young adults in southeast Michigan, in which the NIMH-DIS, revised according to DSM-III-R, was used. Lifetime prevalence of nicotine dependence was 20%. Males and females with nicotine dependence had increased odds for alcohol and illicit drug disorders, major depression, and anxiety disorders, compared with nondependent smokers and nonsmokers combined. Major depression and any anxiety disorder were associated specifically with nicotine dependence. Increased odds for alcohol or illicit drug disorders were observed also in nondependent smokers, compared to nonsmokers. History of early conduct problems increased the odds for nicotine dependence among smokers. Potential mechanisms in the comorbidity of nicotine dependence are discussed.