As global climate change became a serious topic of research in the 1980s, the need for a high-quality, "representative" data set for U.S. climate became apparent. Many of the stations in the NOAA Cooperative Network have data of questionable utility for assessing long-term climate change due to problems of various kinds, such as: (1) data contamination due to the “urban heat island” effect; missing data; change in instruments; change in location; change in surroundings. In an effort to provide a long-term high-quality data set which can be used to assess large-scale climate change in the U.S., the National Climatic Data Center (NCDC) developed the U.S. Historical Climatology Network (HCN)data set. Revision 3 of HCN contained data through 1994. In 2001, an updated version of HCN was released, with data through 1999. Significant changes were made in the data set compared with previous versions. To analyze the changes for Oregon stations, data sets were analyzed station by station. Mean annual temperature (the average of the monthly values) trends were compared for the two data sets. Significant differences were noted, especially in the earlier parts of the record. The earlier HCN data set showed relatively little warming at Oregon rural stations, with virtually no change since the 1930s. The most recent HCN data set, however, has been adjusted significantly. This causes linear trends at most stations to be a great deal higher than they were previously.