The effect of a buoyant boundary layer spawned by a heated vertical plate on the natural convection heat transfer from an upper colinear vertical plate has been determined analytically. The interplate spacing was varied parametrically, as were the relative temperatures and relative lengths of the two plates; the Prandtl number was equal to 0.7 for all cases. Heat transfer at the upper plate was found to be affected both by the preheating and by the finite velocity imparted to the fluid by the first plate, respectively tending to degrade and to enhance the heat transfer. The upper-plate heat transfer was compared to that of an otherwise identical vertical plate, but with the lower plate absent. When the temperatures of the upper and lower plates are the same, the overall upper-plate heat transfer is less than that of its single-plate counterpart for small interplate spacings, with the opposite relationship at larger spacings. If the temperature of the upper plate is substantially below that of the lower plate, the overall heat transfer is degraded. On the. other hand, heat transfer enhancement generally occurs when the upper plate is relatively hot. In general, the heat transfer from relatively short upper plates is very sensitive to the presence of the lower plate, with a lessening sensitivity with increasing plate length. The computed temperature and velocity profiles demonstrated that near the leading edge of the upper plate, a new boundary layer develops within the already existing boundary layer spawned by the first plate.