BACKGROUND: Several epidemiological studies demonstrate associations between high summer temperatures and increased mortality. However, the quantitative implications of projected future increases in temperature have not been well characterized. OBJECTIVE: This study quantifies the effects of projected future temperatures on both mortality and morbidity in California, including the potential effects of mitigation. DATA AND METHODS: We first estimated the association between temperature and mortality for populations close to weather stations throughout the state. These dose-response estimates for mortality were then combined with local measures of current and projected changes in population, and projected changes in temperature, using a baseline of average temperatures from 1961 to 1990, for the years 2025 and 2050. The latter were based on two greenhouse gas emissions scenarios (A2 and B1) developed for the Intergovernmental Panel on Climate Change. In addition, we assessed the impacts of future adaptation through use of air conditioners. Several sensitivity analyses were conducted to determine the likely range of estimates. RESULTS: These analyses indicate that for the high emissions scenario, the central estimate of annual premature mortality ranges from 2100 to 4300 for the year 2025 and from 6700 to 11,300 for 2050. The highest estimates are from the models that use age-specific dose-response functions, while the low estimates are from the models that adjust for ozone. Estimates using the low emissions scenario are roughly half of these estimates. Mitigation based on our estimates of the effects of 10% and 20% increase in air conditioner use would generate reductions of 16% and 33% in the years 2025 and 2050, respectively. CONCLUSION: Our estimates suggest significant public health impacts associated with future projected increases in temperature.