The beginning of residency training for new interns has long been rumored to result in preventable harm for patients, a phenomenon known as the "July Effect" in the US and by the more macabre term "August killing season" in the UK. However, prior studies have reached conflicting conclusions about whether the "July Effect" truly exists. This systematic review of 39 studies provides the first comprehensive evidence that being hospitalized in July may actually be harmful, as a subset of larger and higher quality studies did find that mortality increased and efficiency of care decreased in association with new residents assuming their duties. Unfortunately, most studies included in the review had methodological flaws, meaning that the exact degree of harm could not be quantified.