Four cases of malaria are described. Five to nine days elapsed between the onset of clinical symptoms and the beginning of the anti-malaria chemotherapy. One of the patients died under signs of massive and nearly synchronous hemolysis during the last two hours of treatment. The Malaria tropica disease of the first and the second patient were first diagnosed as common cold with enteritic symptoms; the third patient showed signs of abdominal thyphoid fever and developed symptoms of intravascular coagulation. With the fourth case-Malaria tertiana-it shall be remembered that, if chemoprophylaxis starts too late, the disease is not suppressed but can be delayed: incubation period lasted ten weeks. The diagnostic procedure for the clinician and the pathogenetic mechanism are discussed: massive hemolysis and the possible influence of the chloroquine therapy, the intravascular coagulation and the elevation of the IgE level.