BACKGROUND: Studies of whether inpatient mortality in US teaching hospitals 
rises in July as a result of organizational disruption and relative inexperience 
of new physicians (July effect) find small and mixed results, perhaps because 
study populations primarily include low-risk inpatients whose mortality outcomes 
are unlikely to exhibit a July effect.
METHODS AND RESULTS: Using the US Nationwide Inpatient sample, we estimated 
difference-in-difference models of mortality, percutaneous coronary intervention 
rates, and bleeding complication rates, for high- and low-risk patients with 
acute myocardial infarction admitted to 98 teaching-intensive and 1353 
non-teaching-intensive hospitals during May and July 2002 to 2008. Among 
patients in the top quartile of predicted acute myocardial infarction mortality 
(high risk), adjusted mortality was lower in May than July in teaching-intensive 
hospitals (18.8% in May, 22.7% in July, P<0.01), but similar in 
non-teaching-intensive hospitals (22.5% in May, 22.8% in July, P=0.70). Among 
patients in the lowest three quartiles of predicted acute myocardial infarction 
mortality (low risk), adjusted mortality was similar in May and July in both 
teaching-intensive hospitals (2.1% in May, 1.9% in July, P=0.45) and 
non-teaching-intensive hospitals (2.7% in May, 2.8% in July, P=0.21). 
Differences in percutaneous coronary intervention and bleeding complication 
rates could not explain the observed July mortality effect among high risk 
patients.
CONCLUSIONS: High-risk acute myocardial infarction patients experience similar 
mortality in teaching- and non-teaching-intensive hospitals in July, but lower 
mortality in teaching-intensive hospitals in May. Low-risk patients experience 
no such July effect in teaching-intensive hospitals.