Following an earlier study documenting the relationships between wood anatomical features and climate, we examine the utility of dicotyledonous wood assemblages in assessing paleoclimate. The wood anatomy of dicotyledons from modern forest sites in North, Central, and South America, England, Africa, Malaysia, and the Pacific islands was used to derive equations to predict site mean annual temperature. The best equations gave estimates within 5 degrees C at validation sites. Because trees are physiologically active over a wide range in temperature throughout the year, it would be surprising if wood anatomical characters were more finely tuned to temperature. Mean annual temperature (MAT) was estimated from five fossil wood assemblages: Yellow-stone, Wyoming (early Eocene); Clarno Nut Beds, Oregon (middle Eocene); Post, Oregon (late Eocene); Vantage, Washington (middle Miocene); and Fejej, Ethiopia (Miocene). We used the two best equations to estimate paleotemperature at each site; the MAT estimates differed by 0.1 degrees C at Fejej and 5.7 degrees C at Yellowstone, with differences intermediate to these at the other fossil sites. Compared to present-day values, the paleotemperature estimates indicate that, at middle latitudes of the Northern Hemisphere, mean annual temperatures were warmer in the Eocene and Miocene, whereas the near-equatorial Fejej site was the same temperature in the Miocene as it is today. Where possible, we compared the MAT estimates obtained using fossil woods with those obtained using fossil leaves, and found discrepancies of up to 13 degrees C. These differences may, in part, reflect the complicating effect of evolutionary trends in wood anatomical characters that may have been independent of climate change.