Environmental variables largely determine the disease pattern in developing countries. Infections and malnutrition predominate, due to the effects of both poverty (a summary of many aspects of material deprivation) and a high ambient temperature. Environmental changes may be intended to improve health-examples include improved domestic water supplies and sanitation-or they may be due to socio-economic developments, which often have favourable or unfavourable health consequences. These are explored for water resource developments, where the health effects are complex, and for deforestation and urbanization. Although environmental impact assessment has been of value in reducing the adverse health impacts of socio-economic development projects, the use of health opportunity assessment is proposed as a more positive approach to optimizing the consequences of development to human health.