Despite the growing amount of data, much information is needed on patients' mental capacity to consent to psychiatric treatment for acute mental disorders. The present study was undertaken to compare differences in capacity to consent to psychiatric treatment in patients treated voluntarily and involuntarily and to investigate the role of psychiatric symptoms, competency, and cognitive functioning in determining voluntariness of hospital admission. Involuntary patients were interviewed with the MacArthur Competence Assessment Tool for Treatment (MacCAT-T), the 24-item Brief Psychiatric Rating Scale (BPRS), the Mini Mental State Examination (MMSE) and the Raven's Colored Progressive Matrices, and their data were compared with those for age- and sex-matched voluntary patients. Involuntary patients performed worse in all MacCAT-T subscales. Capacity to consent to treatment varied widely within each group. Overall, involuntary patients have worse consent-related mental capacity than those treated voluntarily, despite capacity to consent to treatment showing a significant variability in both groups.