OBJECTIVES: To characterize excess mortality during a major heat wave in California and its regions; to assess the validity of a simple method. METHODS: We calculated mortality rate ratios for the heat-wave period, using a reference period of the same number of days from the same summer. We conducted alternative analyses and compared our results with those from a time-series model. RESULTS: We estimated 655 excess deaths, a 6% increase (95% confidence interval, 3-9%), impacting varied geographic/climate regions. Alternate analyses supported model validity. CONCLUSIONS: California experienced excess heat-wave related mortality not restricted to high heat regions. As climate change is anticipated to increase heat events, public health efforts to monitor effects assume greater importance.