Many researchers have claimed to find a link between temperature and aggression; we use U.S.A. data to confirm strong seasonal patterns in several types of violent crime. We also report seasonal patterns in U.S.A. workplace data (strikes, and quitting jobs). We suggest a medical explanation for these seasonal patterns, based on stress hormones (adrenaline, and perhaps noradrenaline and/or testosterone). The human body generates adrenaline in response to excessive heat; adrenaline is helpful in keeping the body within safe limits, but we think that as a side effect it leads to aggression (which is often inappropriate). We examine the shape of the curve relating temperature to aggression. Copyright © 2004 John Wiley & Sons, Ltd.