Least-cost health strategies designed to attack the world's leading causes of unnecessary death are explored. Section 1 emphasizes the value of primary health-care procedures--midwifery, maternal education on breastfeeding and weaning, vaccinations, oral rehydration of victims of diarrhea, and antibiotics against respiratory infections--in lowering infant and child mortality in developing countries. Part 2 links the availability of clean drinking water and human waste disposal to diarrheal, tropical, and parasitic diseases. Barriers preventing sanitation development in developing countries are outlined and cost figures are presented. Parts 3 and 4 suggest that the toll of heart disease and cancer in middle age can probably be halved with diet modification and the control of smoking. Educational campaigns for reducing fat and cholesterol consumption, coupled with taxes on tobacco and restrictions on public smoking, are recommended as a way of extending millions of lives into old age at favorable costs. Section 5 offers the best hope of low-cost cures to high-cost diseases such as malaria, sleeping sickness, and the cancers and diseases of the heart. The document concludes that the implementation of the above strategies deserves a high priority because they will effectively and cheaply save the largest number of lives. Eleven data tables supplement the document. (LH)