Background: Inpatient mental health settings have been described as “obesogenic”, due to factors including psychotropic medication, high calorie food provision, restricted physical activity and sedentary lifestyles. No research has investigated the prevalence of obesity among forensic learning disability inpatients, despite this population’s increased risk. Materials and Methods: The weight and Body Mass Index (BMI) data of 46 inpatients (15 women and 31 men) within a specialist learning disability forensic service was examined on, and during admission. Results: Only 13% of inpatients were a normal weight at admission, whereas 87% were overweight or obese. During admission, 61% gained weight, and 2% maintained. However, 37% lost weight, although many remained overweight / obese. Women gained more weight during their admission, but were also more successful in losing weight. There was no correlation between length of stay and weight. Conclusions: The results highlight the need for effective, gender sensitive weight management interventions within similar services nationally.