Weather is one of the key factors controlling crop growth and development. To support decision making, it is essential to know how often extreme weather events have affected crop production and the weather indices that cause them. We used long-term experiment data at four locations in Sweden to evaluate the effects of extreme weather on four major cereal crops: winter wheat, spring wheat, barley and oats. Yield reductions during 1965-2010 differed between crops and locations; with greater variation for spring cereals than winter wheat. For about 2-8 years and 1-2 years, out of the 45 years, yield reductions were 30% and 50%, respectively. For these years the total precipitation during early growth and/or harvest time deviated more than 30% from normal more often, than for years with yield reductions less than 30% (or higher yields). However, such deviations in precipitation were common for the whole 46 year period, and using these weather indices as single predictors of yield reductions would fail in the majority of years.