BACKGROUND: Our aim was to examine whether comorbid mood and anxiety disorders influence patterns of treatment or the perceived unmet need for treatment among those not receiving treatment for illegal drug use disorders. METHODS: Data came from the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC, 2001-2002 and 2004-2005, n = 34,653). Lifetime DSM-IV illegal drug use disorder (abuse and dependence), as well as comorbid mood (major depression, dysthymia, manic disorder, hypomanic disorder) and anxiety disorders (panic disorder, agoraphobia, social phobia, specific phobia, generalized anxiety) were ascertained by a standardized psychiatric interview. Treatment for illegal drug use disorders and perceived unmet need for treatment were assessed among individuals with illegal drug use disorder. Odds of treatment and odds of perceived unmet need for treatment were assessed using logistic regression, adjusting for socio-demographic characteristics, treatment for mood and anxiety disorders, and comorbid alcohol use disorder. RESULTS: Out of 34,653 participants, 1114 (3.2%) had a diagnosis of lifetime illegal drug use disorder: 21.2% had a comorbid mood disorder only, 11.8% a comorbid anxiety disorder only, and 45.9% comorbid mood and anxiety disorders. Comorbid mood and anxiety disorders were not related to treatment for illegal drug use disorders but were associated with an elevated likelihood of unmet need for treatment: compared to participants with no comorbidities, multivariate ORs were 2.21 (95% CI: 1.23- 4.10) for mood disorder only, 2.38 (95% CI: 1.27-4.45) for anxiety disorder only, and 2.90 (95% CI: 1.71-4.94) for both mood and anxiety disorders. CONCLUSIONS: Individuals with an illegal drug use disorder and comorbid mood or anxiety disorders are disproportionately likely to report unmet need for treatment. Integrated mental health and substance use programs could prove effective in addressing their treatment needs.