Background: In the general population, the transition from adolescence to adulthood has been identified as a "high-risk" period for weight gain. There has been no research examining health behaviours over this transition in adults with intellectual disabilities. Methods/design: The feasibility of recruitment, retention and relevant health behaviours were measured in 31 adolescents with mild-moderate intellectual disabilities. Anthropometric, objective physical activity, dietary and self-determination measures were collected over a 12-month transitional period from school to adulthood. Results: Key results suggest weight and BMI increased significantly from month 6 to month 12 (p = 0.044 and p = 0.043). Waist circumference increased significantly from baseline to month 12 (p = 0.049), and from month 6 to month 12 (p = 0.03). Discussion: Recruiting and retaining young adults with intellectual disabilities over a 12-month health behaviour study is feasible. The data indicate the transition from school to adulthood may be the start of a high-risk period for weight gain.