In this study, we evaluate the importance of the mean earlywood vessel size of oaks as a potential proxy for climate in mesic areas. The study was conducted in Switzerland at three forest sites dominated by oak (Quercus petraea and Q. pubescens). The three sites were in different climatic zones, varying mainly in terms of precipitation regime. Three 50-year-long site chronologies of mean earlywood vessel size and tree-ring widths were obtained at each site and related to monthly meteorological records in order to identify the main variables controlling growth. The responses of mean vessel size to climate were compared with those of the width variables to evaluate the potential climatic information recorded by the earlywood vessels. The results show that the mean vessel size has a different and stronger response to climate than ring-width variables, although its common signal and year-to-year variability are lower. This response is better in particular at mesic sites, where it is linked to precipitation during spring, i.e. at the time of vessel formation, and is probably related to the occurrence of only a few processes controlling vessel growth, whereas radial increment is controlled by multiple and varying factors. The mean earlywood vessel size of oak appears to be a promising proxy for future climate reconstructions of mesic sites, where radial growth is not controlled by a single limiting factor.