To analyse the relationship between body mass index (BMI) in middle-age and disability status in old-age using data from the Melbourne Collaborative Cohort Study (MCCS).Methods:A total of 41 514 participants enroled in the MCCS between 1990-1994. Height and weight were measured at baseline and disability, defined as limitations to self-care activities of daily living (ADLs) and self-care plus mobility activities, was identified at follow-up (2003-2007). In all, 6300 participants were 65 years at baseline, 70 years at follow-up and not missing BMI at baseline or ADLs at follow-up. The association between BMI in six categories (BMI 18.5-22.5; 22.5-25; 25-27.5; 27.5-30; 30-35; 35) and disability status was analysed using logistic regression. Models were stratified by sex, and sequentially adjusted for age, education, country of birth, then smoking, alcohol, fruit and vegetable intake, and physical activity.Results:Adjusted odds ratios for composite self-care ADL and mobility limitations compared with BMI 18.5-22.5 kg m-2 were 1.73 (95 CI 1.14-2.64) for BMI 30-35 kg m-2 and 3.46 (1.78-6.73) for BMI 35 kg m-2 in males. In females, adjusted odds ratios were 1.29 (1.00-1.68) for BMI 22.5-25 kg m-2, 1.74 (1.35-2.24) for BMI 25-27.5 kg m -2, 2.58 (1.98-3.36) for BMI 27.5-30 kg m-2, 2.74 (2.10-3.58) for BMI 30-35 kg m-2 and 4.21 (3.12-5.88) for BMI 35 kg m-2.Conclusion:A graded relationship was observed between BMI and disability in males and females, across the continuum of BMI. These results highlight the importance of a healthy body weight at middle age in order to reduce the risk of disability in old age.