In Zambia, an estimated 20% of HIV-positive clients continue to present for first-time antiretroviral therapy (ART) initiation or re-initiation with advanced HIV disease (AHD). The Zambia Ministry of Health (MOH) and other key stakeholders lack information about the characteristics and behaviors of AHD clients, including how they are defined and diagnosed (e.g. low CD4 count v. clinical condition), their demographic and socioeconomic profiles, their HIV care histories, what services they receive, and their short-term outcomes (achieve viral suppression, remain AHD, disengage from care, die) and the timing of these outcomes. The Retain6 project aims to improve HIV treatment outcomes during clients' first six months on ART, when disengagement from care and mortality are highest. This protocol, called Advanced HIV disease during the first six months on antiretroviral therapy in Zambia (AHD Zambia), describes an observational study that will describe the experiences of clients who are diagnosed with AHD upon ART initiation (or re-initiation) in Zambia. Data collected will include clinical and socioeconomic characteristics, clinical and non-clinical needs, services delivered and received, and clients' and providers' concerns and preferences. The study's overall goal is to provide information to the Zambia MOH, treatment program partners, providers, and other stakeholders to better understand who is presenting with AHD in Zambia, how they are currently managed, and their treatment outcomes after starting ART. This information will be useful in determining interventions and guideline changes that might improve short- and long-term outcomes for AHD patients. The study, which will be conducted in collaboration with the Zambia MOH, will include retrospective file reviews, prospective quantitative surveys with outpatient and inpatient clients, qualitative focus group discussions (FGDs) and interviews with clients, and provider surveys and interviews.