Abstract: ObjectiveTo assess the impact of heat waves on the incidence of hospitalization of urban residents in Guangzhou and Xingning and identify the vulnerable population. MethodsData of daily meteorologic record and hospital admission during 2006 to 2011 were collected from two hospitals of the two cities. Distributed lag model was applied to estimate cumulative effects of heat waves on the incidence of hospitalization and meanwhile analyze the effect of heat waves on the age, sex, and disease category.Results During 2006-2011, the temperature thresholds of heat waves in cities of Guangzhou and Xingning were 30.8 and 30.9 ℃, respectively. The total number of heat wave days, intensity of heat waves (mean daily temperatures during heat waves), and average number of days for continuous heat waves in Xingning were higher than those in Guangzhou. Heat waves occurred from June to September in both cities. The effect of the heat wave on hospitalization in the two cities was relatively short and was the greatest at lag-0 day (RR: 1.045, 95%CI: 1.005-1.165 in Guangzhou; RR: 1.165, 95%CI: 1.142-1.288 in Xingning). The cumulative effects of heat waves on hospitalization were largest at a lag of 0-1 day; the number of inpatients increased by 2.6% (95% CI: 1.4%-3.8%) in Guangzhou and 4.5% (95% CI: 2.0%-6.0%) in Xingning. Compared with people aged 65 years or below, patients with circulatory system diseases, and male, greater effects were observed in those with respiratory diseases (CER: 4.6%）, aged 75 years and older (CER:5.7%), and female（CER: 4.7%）in Xingning. ConclusionThe heat wave increased the risk of hospital admissions in Guangzhou and Xingning. Respiratory diseases, people aged 65 and over, and women are vulnerable to the heat waves.