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.