Through prospective enrollment of canine patients at the Ross University Veterinary Clinic, on St. Kitts, four cases of acute fatal leptospirosis were diagnosed. Clinical, pathological, and diagnostic findings in these cases are summarized in this case series. Icterus, thrombocytopenia, hyperphosphatemia, pulmonary hemorrhage, and both hepatocellular and renal damage were noted in all four cases. Interestingly, extensive myocardial involvement was also observed in one case. To our knowledge, this is the first documentation of myocarditis in a dog with leptospirosis, and the first report of fatal leptospirosis in any animal species on the Caribbean island of St. Kitts.