Research on the quality of medical care is needed for evaluating both the quality of medical care and its cost. Quality-of-care research for human immunodeficiency virus (HIV) illness is complicated by several factors, including the multiplicity of clinical manifestations, diversity of affected populations, rapid changes in modified natural history and in treatment approaches, and problems of access to care. Researchable questions can be formulated by identifying sizable subgroups of patients that are relatively homogeneous with respect to risk of adverse outcomes and issues of clinical management. Four such groups are patients with early HIV disease, patients with major opportunistic infections, patients with acquired immunodeficiency syndrome dementia complex, and patients in the terminal stages of illness. For each group, the major research issues concern the appropriate settings, resources, and processes of care and the relevant outcomes, adjustors, and sources of data. Because specific therapies are still undergoing rapid change, research on processes of care that are likely to be important across a range of specific therapies may yield results that are more robust over time. A focus on defined systems of care and on episodes of care that encompass both inpatient and outpatient management may offer a better picture of quality of care than that provided by studying only the inpatient phase of care.