BACKGROUND Each July thousands begin medical residencies and acquire increased responsibility for patient care. Many have suggested that these new medical residents may produce errors and worsen patient outcomes—the so-called “July Effect;” however, we have found no U.S. evidence documenting this effect. OBJECTIVE Determine whether fatal medication errors spike in July. DESIGN We examined all U.S. death certificates, 1979–2006 (n = 62,338,584), focusing on medication errors (n = 244,388). We compared the observed number of deaths in July with the number expected, determined by least-squares regression techniques. We compared the July Effect inside versus outside medical institutions. We also compared the July Effect in counties with versus without teaching hospitals. OUTCOME MEASURE JR = Observed number of July deaths / Expected number of July deaths. RESULTS Inside medical institutions, in counties containing teaching hospitals, fatal medication errors spiked by 10% in July and in no other month [JR = 1.10 (1.06–1.14)]. In contrast, there was no July spike in counties without teaching hospitals. The greater the concentration of teaching hospitals in a region, the greater the July spike (r = .80; P = .005). These findings held only for medication errors, not for other causes of death. CONCLUSIONS We found a significant July spike in fatal medication errors inside medical institutions. After assessing competing explanations, we concluded that the July mortality spike results at least partly from changes associated with the arrival of new medical residents.