This research focused on factors associated with schizophrenia as well as on the feasibility and effectiveness of group metacognitive training (MCT) in schizophrenia patients with a history of violence in a high-security forensic hospital setting. One of the aims was to scrutinize reasoning ability in regards to jumping to conclusions (JTC) cognitive bias and the expression of cognitive insight (insight into one’s own thinking), a metacognitive ability. A second aim was to examine the associations of these factors with each other and with insight into illness and delusions. Moreover, the patients’ health-related quality of life (HRQOL) was explored. A further objective was to study the potential effects of MCT on delusions, overall severity of illness, reasoning and HRQOL. The patients’ subjective assessments of MCT were also included. A cross- sectional study (n = 20) was performed to examine the selected characteristics and their associations. It also served as a baseline assessment for two randomised controlled trials (RCT) where the patients were randomised either to eight-session MCT or treatment-as-usual control group. The measurements were also made immediately following MCT and at three and six months afterwards. The patients’ HRQOL was also compared with a general population. The results revealed that 75% of the patients made hasty conclusions (JTC). The more data a patient gathered, the more insight into illness he had and the less distressed he was by his symptoms. The results also indicate that group MCT may reduce symptomatology, especially suspiciousness. The greatest benefit was seen after three months. MCT was also highly accepted and appreciated by the patients. The patients’ HRQOL was significantly worse than that of the general population and MCT did not have any impact on it. In the treatment, it may be useful to evaluate all of the factors covered in this research in order to achieve a more comprehensive and individualized approach to the individuals’ situation. MCT is a noteworthy rehabilitation method in forensic psychiatry but the process of change takes time and more extensive training is suggested. Additionally, other means than those aimed at managing symptoms should be utilised to improve the patients’ HRQOL.