Background <p>Forensic psychiatric patients have a reduced life expectancy, largely due to cardiovascular diseases. Low maximal oxygen uptake is a risk factor for cardiovascular diseases in the general population and may also be essential to the cardiovascular risk among forensic psychiatric patients. The purpose of this study was to verify previous results of very low estimated maximal oxygen uptake levels in a larger group of forensic psychiatric patients and investigate the impact of the length of inpatient forensic psychiatric care on estimated maximal oxygen uptake. Further, we examined the extent to which BMI, level of physical activity, and smoking status were associated with estimated maximal oxygen uptake.</p> Methods <p>We evaluated estimated levels of maximal oxygen uptake based on clinical testing of 115 forensic psychiatric patients and the development of these levels during inpatient care for those who underwent retesting (two tests, n = 66, three tests, n = 30).</p> Results <p>Mean estimated levels of maximal oxygen uptake were remarkably low, confirming previous findings. Levels correlated negatively with higher body mass index, lower physical activity, and older age; however, no change was observed during forensic psychiatric inpatient care. Estimated physical activity levels tended to increase between tests one and two, but decreased significantly between tests two and three.</p> Conclusions <p>This Swedish forensic psychiatric cohort had very low cardiovascular fitness. The strongest associated factors were a high BMI and a low level of physical activity. These results make essential contributions to the planning of future studies on treatment strategies aimed at improving metabolic health in forensic psychiatric patients. The importance of aerobic exercise, as well as means to encourage patients to reach and maintain recommended levels of physical activity, needs to be further explored.</p>