All 50 US states allow medical exemptions from school entry immunization requirements. The extent to which medical exemptions are granted and the relationship with ease of obtaining these exemptions has not previously been examined in detail. We evaluated counts and rates of state-level medical exemptions to kindergarten entry requirements over 7 school years (2004-2005 through 2010-2011). During this period, 0.26%-0.41% of enrolled children received medical exemptions. In states with easier medical exemption criteria, medical exemption rates were significantly higher (adjusted incidence rate ratio: 6.4 [95% confidence interval: 2.7-15.6]). Routine evaluation of medical exemption rates is needed to ensure their appropriate use.