Average upper mantle Pn velocities and heat flow were computed within continental physiographic provinces in North America from published data. Pn velocity and heat flow data display an inverse relationship and were found to correlate with a statistically significant correlation coefficient. Temperatures at the crust-mantle boundary were estimated from the heat flow values, and these were used to demonstrate a correlation between Pn velocity and temperature of upper-mantle material. The value of (∂Pn/∂T)p thus obtained (-4.4¿10-4 to -8.1¿10-4 km s-1 ¿C-1) is within the range of temperature derivatives determined from laboratory studies of ultramafic rocks. This dependence of Pn velocity on temperature implies that one possible explanation for the observed geographical distribution in upper-mantle seismic velocity is that the variation in Pn velocity is primarily a temperature effect. Combined with the relationship between heat flow and crustal age for continents, the Pn versus heat flow correlation suggests a relationship between Pn velocity and crustal age, probably due to progressive cooling of the continental lithosphere after a thermotectonic event.