Twenty-three patients with cardiac failure were observed clinically at rest in neutral (75 F, 41% relative humidity [RH]), hot, dry (90 F, 41% RH), and hot, humid (90 F, 75% RH) environments. Fifteen patients developed clinical evidence of overt congestive heart failure in the hot, dry climate; 18 developed symptoms in the hot, humid environment, two having episodes of angina. Seventeen patients displayed T wave changes in their electrocardiograms in both hot environments, and nine showed significant changes in their vectorcardiograms. Venous pressure increased in both hot environments in 12 patients, decreased in six in the hot, dry environment, and decreased in five in the hot, humid atmosphere. Body temperature, pulse, respiratory rate, and blood pressure increased in the heat in most patients. The hot, humid environment was less tolerable than the hot, dry one and imposed a greater burden on the cardiovascular system, especially in patients with the most