Cochrane) were searched to detect any randomized controlled trial (RCT) pertinent to conventional exercise trainings for adults with ID. Meta-analyses were being performed for studies reported results on the same outcome measures employing RevMan 5.3. Thirteen RCTs involving 556 participants (56.7% men) entered the qualitative synthesis and eleven the meta-analyses. The effect of exercise was superior to controls (no exercise prescription) on several cardiovascular variables (peak oxygen uptake, MD 2.89, 95%CI 0.35;5.42, I 2 = 69%; heart rate peak, MD 4.64, 95%CI 2.15;7.14, I 2 = 7%; maximal exercise capacity, SMD 0.55, 95%CI 0.03;1.08, I 2 = 67%; systolic blood pressure, MD −9.62, 95%CI −17.07;-2.18, I 2 = 40%), without differences in diastolic blood pressure and total cholesterol. None of the anthropometric and body composition variables, six-minute walking test distance and handgrip strength values revealed significant changes at the follow-up. In adults with IDs, conventional exercise training substantially improves cardiovascular fitness and exercise capacity, while it seems of limited effectiveness on anthropometric and body composition variables.