This study aims at comparing mentally disordered offenders and general psychiatric patients regarding violent ideation and at exploring its association with interpersonal violence. We recruited 200 detainees undergoing forensic psychiatric evaluation and 390 general psychiatric patients at discharge. At baseline, they were asked about violent ideation; at the 20-week follow-up, information about violent acts was gathered from crime conviction registry, interviews, and records. The lifetime prevalence of violent ideation was 32.5% for offenders and 35.6% for patients; the corresponding two-month prevalence was 22.5% and 21.0%, respectively. For the both samples combined, those with violent ideation in their lifetime were significantly more prone to commit violent acts during follow-up than those without such ideation, OR = 2.65. The same applied to the patient sample, OR = 3.41. In terms of positive predictive values, fewer than 25% of those with violent ideation committed violent acts. Contrary to our hypothesis, the prevalence of violent ideation did not differ significantly between offenders and patients. However, there was support for the hypothesized association between violent ideation and violent acts on a group level. On an individual level, the clinician should consider additional factors when assessing the risk for violent acts.