Experiments were performed to study the interactive natural convection from a pair of heated horizontal cylinders situated one above the other in a vertical plane. Prime attention was focused on how the heat transfer characteristics of the upper cylinder are affected by the presence of the lower cylinder. The vertical center-to-center separation distance between the cylinders was varied from two to nine cylinder diameters. The cylinderto- cylinder temperature imbalance was also varied independently and systematically, with the wall-to-ambient temperature difference for the lower cylinder ranging from zero to three times that for the upper cylinder. Experiments were carried out for upper-cylinder Rayleigh numbers from 20, 000 to 200, 000. It was found that for a given temperature imbalance and upper-cylinder Rayleigh number, the upper-cylinder Nusselt number takes on a maximum value as a function of separation distance. The separation distance for which the maximum occurs is in the range of seven to nine cylinder diameters. The enhancement or degradation of the upper-cylinder Nusselt number relative to that for a single cylinder is strongly dependent on the separation distance, with degradation of the Nusselt number being more common at small separations and enhancement prevailing at larger separations. With regard to the temperature imbalance, its effect on the Nusselt number is of major importance at small separations but not at large separations.