Little is known about the ability of persons with severe mental disorders to give consent to sexual activity. A possible reason for this shortcoming is the absence of specific criteria and tools to measure sexual consent in psychiatric clinical settings. We developed a clinician oriented semi-structured interview, the Sexual Consent Assessment Scale (SCAS), and investigated sexual consent capacity in a sample of hospitalized patients with bipolar disorder (n = 54, M (age) = 38.1 years, 48% males) and schizophrenic spectrum disorders (n = 31, M (age) = 38.4 years, 29% males). The SCAS items were derived from the criteria proposed by Kennedy and Niederbuhl (Am J Ment Retard, 106:503-510, 2001). The full scale and a shorter scale comprising 10 items (SCAS-10) achieved good initial validity. Patients with schizophrenic spectrum disorders had worse sexual consent capacity than patients with bipolar disorder. This difference was unexpectedly independent from patients' symptomatology, as measured by the Brief Psychiatric Rating Scale. Conversely, poor cognitive functioning measured by the Raven's Standard Progressive matrices was associated with reduced capacity to give sexual consent in both groups. Subjects in the schizophrenic spectrum disorders group were more frequently judged incapable in basic knowledge of birth control methods and in domains underlying metacognitive abilities. Principal component analysis revealed two SCAS-10 interpretable factors: "appropriateness-recognition" and "consequences-metacognition." Our study suggests that patients with severe psychiatric disorders, especially those with cognitive dysfunction, might be at risk of incapacity to give valid sexual consent.