Background: Adults with mental retardation (MR) have been identified as being more sedentary than those without disabilities based on (a) proxy reports of physical activity (PA) and (b) physiological measures such as body composition or cardiovascular fitness. However, there exist no objective, direct comparisons of PA levels between adults with and without MR. The purpose of this study was to compare physical activity (PA) levels between adults with and without MR using accelerometry. Methods: Twenty-two adults with MR (11 men, 11 women; age = 34.9 ± 9.0 y), 17 sedentary controls without MR (SC; 8 men, 9 women; age = 35.8 ± 7.6 y), and 9 active controls without MR (AC; 5 men, 4 women; age = 34.1 ± 5.8 y) wore a Manufacturing Technology Inc (model 7164) accelerometer for 7 d. Data were collected in 1-min epochs and categorized according to light, moderate, hard, and very hard PA. Differences between groups on dependent measures were examined using a 1-way ANOVA. Results: Both MR and SC groups were less active (F2,47 = 12.17, P = 0.00, hp2 = 0.35), engaged in less moderate-hard PA ((F2,47 = 11.28, P = 0.00, hp2 = 0.33), and engaged in fewer bouts of moderate-hard, continuous PA (F2,47 = 11.71, P = 0.00, hp2 = 0.34) than AC subjects. There were no differences between MR and SC subjects on the variables measured. Conclusions: The results suggest that adults with MR exhibit PA patterns similar to sedentary adults without MR. Interventions for this population should target participation in continuous, moderate PA.