Background There is relative little information about the prevalence and risk factors of co-morbid anxiety and depression in later life. These disorders are often associated with worse response to treatment than either condition alone, and researching their epidemiology in diverse settings is vital to policy makers. We therefore investigated the co-occurrence of anxiety and depressive syndromes amongst older adults living in developing countries and measured the separate and joint effect of these two disorders on levels of associated disability.Method The 10/66 study carried out cross-cultural surveys of all residents aged 65 years or over (n=15021) in 11 sites in seven countries (People's Republic of China, India, Cuba, Dominican Republic, Venezuela, Mexico and Peru). Anxiety was measured by using the Geriatric Mental State Examination and the Automated Geriatric Examination for Computer Assisted Taxonomy diagnostic system. Depression was assessed according to International Classification of Diseases 10th revision (ICD-10) and EURO-D criteria. Disability was measured by using the World Health Organization's Disablement Assessment Scale Version II. Zero-inflated negative binomial regression models were used to investigate the association of common mental disorders and disability.Results The prevalence of co-occurring anxiety and depression (with the exclusion of subthreshold disorders) ranged between 0.9% and 4.2% across sites. Gender, socio-economic status, urbanicity and physical co-morbidities were associated with the different co-morbid states. Having both disorders was linked to higher disability scores than having anxiety or depression alone.Conclusions Given the close association of co-morbid anxiety and depression with disability, new policies to improve prevention, recognition and treatment will be needed to adapt to ageing populations and their mental health needs. © Cambridge University Press 2011.