Depression in later life is common, costly, and can have devastating consequences for those affected, their relatives, and society. Notwithstanding this, it usually goes unrecognised and untreated, especially in low-middle income countries. The Brazilian population is ageing rapidly, with already 20 million people aged 60 or more. The Brazilian health care system, especially the mental health sector, is poorly prepared to meet this challenge. There is an urgent need to develop cost-effective depression treatment programmes for older people living in low- and middle-income countries. The Brazilian primary care system is an excellent setting to introduce and evaluate an intervention to reach a large proportion of this neglected part of the population. The intervention proposed (PROACTIVE) aims to overcome barriers for treating old people with depression, such as patients' social isolation and difficulties in accessing services, lack of skilled and supported staff to deliver effective interventions, and poor coordination and accountability among staff caring for elderly people. PROACTIVE will be a two-arm cluster randomised controlled trial aiming to compare the effectiveness and cost-effectiveness of adding to usual care a psychosocial, community-based intervention mostly delivered at home by Community Health Workers employed by the existing primary care system. The intervention will be compared with an 'enhanced' usual care in reducing depressive illness and improving functioning among adults 60 years or older from poor socioeconomic backgrounds in São Paulo, Brazil. Primary Care Family Health Units will be randomised to one of these two treatment groups. All or a random sample of the Family Health Teams within a Unit, depending on size of the Unit, will be invited to participate. PROACTIVE consists of 8 to 11 home sessions, depending on severity of depression, delivered over 17 weeks. The initial phase is given to all participants and comprises three sessions covering psycho-education and learning simple coping strategies to improve mood. The Second Phase is based on behaviour activation and relapse prevention strategies; the number of sessions depends on the severity of symptoms. Community Health Workers will be equipped with tablet computers to assist with the delivery and accountability of the intervention and to receive further training and supervision. Those participants who do not improve with the intervention, in relation to specified clinical algorithms, will be discussed in supervision and regular team meetings and, if needed, other clinical decisions will be adopted. The control group will receive 'enhanced' usual care in so far as improved identification and periodic assessments of high-risk cases treated as per a 'high-risk' protocol. The primary outcome measure of PROACTIVE is the 9-item Patient Health Questionnaire (PHQ-9). We will compare the recovery of cases (PHQ-9<10) across arms at 8 and 12 months after entering the trial, using an intention-to-treat analysis. Several secondary outcomes will be also measured including quality of life and levels of functioning. Direct and indirect costs in both arms will be measured to undertake a cost-effectiveness analysis. We anticipate recruiting a total of 1,440 participants registered in 20 FHUs (clusters), yielding 86.5% power for a 15 percentage point difference (25% to 40%) in recovery at 8 months. We have developed and successfully tested the feasibility and acceptability of the proposed intervention in primary care in São Paulo (RCUK/FAPESP). This project has the potential for a timely and major impact on the wellbeing of depressed older adults, further reducing dependency on specialised mental health resources already under strain in Brazil and most LMIC. The Brazilian primary care model is being replicated in several other LMIC, contributing to increase the portability of this intervention to other LMIC should its cost-effectiveness be demonstrated.