Scientists predict that global warming will have serious effects on public health, from worsening air quality, degradation of food and water supplies, and increasing levels of allergens to heat waves, flooding, and changing patterns of mosquito, tick, and flea-borne disease. Fortunately, early action by city managers, health officials, and the federal government can reduce the impact of these problems on our health by preparing and responding appropriately to the effects of global warming. But we must start now.