Severe behavioral problems, aggression, unlawful behavior, and uncooperativeness make the forensic psychiatric population both hard to treat and study. To fine-tune treatment and evaluate results, valid measurement is vital. The Brief Psychiatric Rating Scale–Extended (BPRS-E) is a widely used scale for assessing psychiatric symptoms, with a stable factor structure over various patient groups. For the first time, its usefulness for forensic psychiatric patients was studied by means of an exploratory factor analysis on 302 patients in a penitentiary psychiatric center. A five-factor solution fitted the data best and showed large overlap with previous research done in both in- and outpatient populations with schizophrenia and mixed diagnoses. Around 45% of the patients did not fully comply. Items relying most on self-report caused the most non-adherence, possibly because of difficulty with verbalizing distress. These items loaded on the factors psychosis and affect. The BPRS-E is a suitable instrument for forensic use. Future research and clinical practice should focus on alignment with forensic patients to improve measurement, understanding, and eventually therapeutic interventions.