The anatomical features of earlywood vessels often reflect information about past climatic conditions. We examined the relationships between mean monthly temperature and mean vessel lumen area (MVA) in various categories of earlywood vessels. Subsets of earlywood vessels of chestnut (Castanea sativa Mill.) were selected from a previously reported dataset based on several progressive size-related procedures. To include all earlywood vessels, the minimum size considered was 10,000 µm². Changes in the correlations between MVA and the mean air temperature in March are described and discussed. The results show that not all vessels embody the same information. The MVA of a proportion of the largest earlywood vessels in each annual ring was most closely related to March temperature, whereas MVA of the smallest earlywood vessels was better correlated with June temperature. This difference is probably a result of the vessels being formed at different times: early spring for the largest earlywood vessels and later in spring for the smallest earlywood vessels. Analyses combining large and small vessels yielded lower correlations between MVA and monthly temperature. The number, size and distribution of vessels can vary greatly from ring to ring. In making year-to-year comparisons, the best information is provided by observations on vessels of contemporaneous ontogenesis. Criteria for the selection and analysis of vessels in the assessment of temperature during the season of wood formation are proposed and discussed.