Open up any biomedical or public health journal prior to the 1970s, and one term will be glaringly absent: gender. Open up any recent biomedical or public health journal, and two terms will be used either: (1) interchangeably, or (2) as distinct constructs: gender and sex. Why the change? Why the confusion?-and why does it matter? After briefly reviewing conceptual debates leading to distinctions between 'sex' and 'gender' as biological and social constructs, respectively, the paper draws on ecosocial theory to present 12 case examples in which gender relations and sex-linked biology are singly, neither, or both relevant as independent or synergistic determinants of the selected outcomes. Spanning from birth defects to mortality, these outcomes include: chromosomal disorders, infectious and non-infectious disease, occupational and environmental disease, trauma, pregnancy, menopause, and access to health services. As these examples highlight, not only can gender relations influence expression-and interpretation-of biological traits, but also sex-linked biological characteristics can, in some cases, contribute to or amplify gender differentials in health. Because our science will only be as clear and error-free as our thinking, greater precision about whether and when gender relations, sex-linked biology, both, or neither matter for health is warranted.