Cognitive impairment has been found independently among individuals with schizophrenia and individuals with alcohol use disorders. Less is known about the nature and severity of cognitive impairment in patients with a dual diagnosis, though the co-occurrence of these disorders may further exacerbate cognitive impairment. The study investigates the possible additive effect of alcohol use disorder and schizophrenia on cognitive impairment among patients diagnosed with schizophrenia. Participants were inpatients with schizophrenia (n =30), inpatients with a dual diagnosis of schizophrenia and alcohol use disorder (n =30), and matched controls (n =30): all completed a comprehensive neuropsychological battery. Both patient groups were significantly impaired, relative to controls, across the battery. Dual diagnosis patients were significantly more impaired than schizophrenia patients on delayed verbal memory, and executive functioning, primarily set-shifting, working memory, and planning, and had higher psychiatric morbidity scores. The findings provide support for an additive effect of the two disorders on cognitive impairment. These cognitive deficits may affect capacity to engage in treatment, increase risk of relapse, and adversely affect treatment outcomes. An understanding of the cognitive profile of people with dual diagnosis may help to tailor treatment delivery to meet their specific needs, enhance cognitive strengths, accommodate deficits and improve treatment outcomes.