Every July, 100,000 house staff change jobs, with the sudden arrival of huge cohorts of promising talented young interns who also happen to be (by definition, and speaking from personal experience) disoriented and incompetent. Does changeover result in excess mortality -- the so-called "July effect" in the U.S. (in the U.K., they go right ahead and call it "the killing season")? The question has been debated for decades. Young et al performed a systematic review of 39 studies published since 2000, 13 of which were determined to be of higher quality. Studies were too heterogeneous to combine into a meta-analysis. The answer is that yes, deaths do go up a little in July. Although many small studies found no difference, the biggest and best-designed studies showed an increase in mortality that ranged from a relative risk increase of 4-12%, which corresponded to an adjusted odds ratio of 1.08 to 1.34. Like I've always said, June is a great month to get a knee replacement.