Comorbidity between a substance use disorder (SUD) and another psychiatric disorder is known as dual diagnosis. It is of great relevance due to its important clinical consequences and costs of care. There are practically no published studies on dual diagnosis prevalence in patients admitted to psychiatric hospitalization units in general hospitals (PHUGH) in our country. The objectives were to estimate the prevalence of dual diagnosis in psychiatric inpatients admitted consecutively to a Psychiatric Hospitalization Unit (Hospital Universitario 12 de Octubre, Madrid, Spain) in one year, to compare clinical and sociodemographic variables between the dual diagnosis group (DD group) and the group with a psychiatric disorder but no SUD (PD group), and to study the types of substances used. This is a retrospective study, based on the review of the clinical charts of the 257 patients admitted to this PHUGH in one year. The results showed that, excluding nicotine dependence, 24.9% of our inpatients had a SUD as well as another psychiatric disorder. A statistically significant predominance of men was found in the DD group, as well as a younger age at the time of the study, at the beginning of their psychiatric attention and on their first psychiatric admission, and they had received diagnoses of schizophrenia or related psychoses more often than the PD group, who had mostly affective disorders. The substances most frequently used in the DD group were alcohol (78.1%), cannabis (62.5%), and cocaine (51.6%). Due to the high prevalence and repercussions of dual diagnosis, it would be advisable to have specialized therapeutic programs for its treatment.