Objectives: Among older people living with HIV (PLWH) and comparable individuals without HIV, we evaluated whether associations of HIV and antiretroviral therapy (ART) with disability depend on body mass index (BMI). Methods: We analyzed 4552 participants in the "Health and Aging in Africa: A Longitudinal Study of an INDEPTH Community in South Africa." (HAALSI) We compared prevalence of disability (>/=1 impairment in basic activities of daily living) by HIV status, ART use, and BMI category, adjusting for age, sex, education, father's occupation, country of origin, lifetime alcohol use, and primary health-care utilization. Results: Among PLWH, those underweight had 9.8% points (95% confidence interval (CI): 1.2 to 18.4) higher prevalence of disability than those with normal BMI. Among ART users, those underweight had 11.9% points (95% CI: 2.2 to 21.6) higher prevalence of disability than those with normal BMI. Conclusions: We found no evidence that weight improvement associated with ART use is likely to increase disability.