There is a need to evaluate the professionals' perception about the consequences of the lack of therapeutic adherence in the evolution of patients with co-occurring disorders.An online survey, released on the Socidrogalcohol [Spanish Scientific Society for Research on Alcohol, Alcoholism and other Drug Addictions] and Sociedad Española de Patología Dual [the Spanish Society of Dual Pathology] web pages, was answered by 250 professionals who work in different types of Spanish health centers where dual diagnosis patients are assisted.Most professionals perceived the existence of noncompliance among dual diagnosis patients. Almost all of these professionals (99%) perceived that noncompliance leads to a worsening of the progression of the patient's disorder, in both the exacerbation of mental disorders and the consumption of addictive substances. Most of the professionals (69.2%) considered therapeutic alliance as the main aspect to take into account to improve the prognosis in this population. The primary purpose of treatment must be the improvement of psychotic-phase positive symptoms, followed by the control of behavior disorders, reduction of craving, improvement of social and personal performances, and reduction of psychotic-phase negative symptoms.Most professionals perceived low adherence among dual diagnosis patients. This lack of adherence is associated with a worsening of their disease evolution, which is reflected in exacerbations of the psychopathology and relapse in substance use. Therefore, we propose to identify strategies to improve adherence.