Research into the climate of the Middle Ages has relied heavily upon data provided by compilations of references to weather and related phenomena extracted from a variety of historical texts and source documents. These compilations, produced from 1858 onwards, have generally neglected the essential need for source validation. While a considerable amount of reliable and useful information about medieval climate is to be found in documentary sources, it occurs together with material which is spurious, inaccurate, or whose reliability cannot be properly authenticated. Because they were, for the most part, scientists, unfamiliar with historical methodology and techniques of source analysis, the authors of the compilations were either unaware of the problematic character of their sources, or ignorant of the techniques developed by historians for dealing with them. The material included in the compilations must be regarded as suspect until its authenticity has been checked by validating individual sources. Unless this is done, a misleading picture of the climate of the Middle Ages may emerge from uncritical use of the compilations. In particular, the climate may appear to have been more extreme than authentic sources alone would suggest.