Prison populations are growing in Western countries. Imprisoned people usually have a poor health status and an increased risk to suffer chronic debilitating conditions as coinfection with the HIV and hepatitis C virus (HCV) and/or opioid dependency. We studied the effects of a 4-month concurrent cardiorespiratory and resistance training program on the cardiorespiratory fitness, lower and upper body dynamic strength endurance (6-RM test for bench press and knee-extensor exercise, respectively), muscle mass and quality of life (QOL) of adult prison inmates who are HIV/HVC co-infected and enrolled in a methadone maintenance program (n = 9; mean [SD] age: 37 [3] yrs). We also evaluated a control group (n = 10; 37 [2] yrs). A significant combined effect of group and time was found for peak completed workload (W) (p < 0.01), peak heart rate (HR (peak)) (p < 0.05) and rate of HR decrease at 1-min postexercise compared to HR (peak) (p < 0.05), respectively, in a gradual cycle ergometer test. A significant combined effect of group and time was also found for both bench press and knee-extensor 6-RM tests, respectively (p < 0.05). Supervised exercise training can improve the overall physical fitness of incarcerated people. Our results suggest that this type of intervention could be applied in prisons of Western societies.