INTRODUCTION: An increasing proportion of adult patients initiating antiretroviral therapy (ART) in resource-limited settings are aged >50 years. Older populations on ART appear to have heightened risk of death, but little is known about factors influencing mortality in this population. METHODS: We performed a retrospective observational multisite cohort study including all adult patients (>/=15 years) initiating ART between 2003 and 2013 in programmes supported by Medecins Sans Frontieres across 12 countries in Asia, Africa and Europe. Patients were stratified into two age groups, >50 years and 15 to 50 years. A Cox proportional hazards model was used to explore factors associated with mortality. RESULTS: The study included 41,088 patients: 2591 (6.3%) were aged >50 years and 38,497 (93.7%) were aged 15 to 50 years. The mortality rate was significantly higher in the age group >50 years [367 (14.2%) deaths; mortality rate 7.67 deaths per 100 person-years (95% confidence interval, CI: 6.93 to 8.50)] compared to the age group 15 to 50 years [3788 (9.8%) deaths; mortality rate 4.18 deaths per 100 person-years (95% CI: 4.05 to 4.31)], p<0.0001. Higher CD4 levels at baseline were associated with significantly reduced mortality rates in the 15 to 50 age group but this association was not seen in the >50 age group. WHO Stage 4 conditions were more strongly associated with increased mortality rates in the 15 to 50 age group compared to populations >50 years. WHO Stage 3 conditions were associated with an increased mortality rate in the 15 to 50 age group but not in the >50 age group. Programme region did not affect mortality rates in the >50 age group; however being in an Asian programme was associated with a 36% reduced mortality rate in populations aged 15 to 50 years compared to being in an African programme. There was a higher overall incidence of Stage 3 WHO conditions in people >50 years (12.8/100 person-years) compared to those 15 to 50 years (8.1/100 person-years) (p<0.01). The rate of Stage 4 WHO conditions was similar (5.8/100 versus 6.1/100 respectively, p=0.52). Mortality rates on ART associated with the majority of specific WHO conditions were similar between the 15 to 50 and >50 age groups. CONCLUSIONS: Older patients on ART in resource-limited settings have increased mortality rates, but compared to younger populations this appears to be less influenced by baseline CD4 count and WHO clinical stage. HIV treatment programmes in resource-limited settings need to consider risk factors associated with mortality on ART in older populations, which may differ to those related to younger adults.