The objective of this study is to assess the prevalence of psychiatric comorbidity in patients under treatment within the addictive disorders assistance units of Galicia (Spain). Material and methods: A total of 64 healthcare professionals performed clinical diagnosis of mental disorders (on DSM IV-TR criteria) in 2300 patients treated throughout March 2010 in 21 addictive disorders assistance units. Results: 56.3% of patients with substance abuse/dependency also showed some other mental disorder, 42.2% of patients suffering from at least an Axis I condition and 20.2% from some Axis II condition. Mood and anxiety disorders and borderline and antisocial personality disorders were the most frequent disorders in both axes. Conclusions: A high comorbidity was found between mental and substance use disorders (SUD) in patients seen at the addictive disorders assistance units of Galicia.