Twenty hearts from patients dying of leptospirosis were studied. Interstitial myocarditis was found in 50% of the cases, and a significant statistical correlation was observed between myocarditis and the inflammatory involvement of the conduction tissue. Acute coronary arteritis, affecting the main branches of the coronary arteries, was observed in 70% of the cases, and this finding also correlates significantly with interstitial myocarditis. Aortitis was found in 57.8% of the cases. When serum against L. interrogans serovar icterohaemorrhagiae was used, focal IP antigen deposits were observed in the coronary arteries and in the aorta. Experimental data from 12 guinea-pigs inoculated with L. interrogans serovar icterohaemorrhagiae showed a focal myocarditis involving mainly the subendocardial and pericoronary heart tissue, with IP antigen deposits in the same sites. Leptospirosis might be visualized as a generalized illness resembling other infectious vasculitides. The heart and main vessels are involved during the septicaemic phase of the disease, and bacterial migration, toxin(s), enzymes and/or antigenic products liberated by bacterial lysis might account for the increased endothelial permeability with antigen deposits and inflammation.