Blackwater fever is probably a pernicious complication of malarial fever, and derives its name from the colour of the urine. It must be remembered that dark-coloured urine is usual in all fevers; it is scanty during the height of the fever, especially if there is much sweating. If, however, it is obviously “bloody,” the case is more grave, but as a rule it is only men broken down in health, and those who have resided in blackwater countries, _e.g._, Tropical Africa, parts of India, the West Indies, etc., for at least a year, especially those who have taken their prophylactic quinine irregularly, who suffer from this complication. The reason for the occurrence of this condition is not fully known. Some have ascribed the symptoms entirely to the taking of quinine, but as the fever often occurs where no quinine has been given this is impossible. At the same time it appears likely that in certain cases of malaria, owing perhaps to some idiosyncrasy, quinine may help to bring on the symptoms. It is possible that chill occurring during the course of a fever may lead to the production of blackwater fever. Those who have had one attack are particularly liable to a recurrence, and after two consecutive attacks return to a temperate climate is required. _Symptoms._—In addition to the ordinary symptoms of malarial fever, the urine is dark, blood-like, and eventually porter-coloured; it is often scanty, and may become entirely suppressed. The skin is yellow, often a bright orange, there is frequent vomiting, often hiccough, and the vomited matter is usually of a green colour. _Prophylaxis._—With our present knowledge all that can be said is that malaria prophylaxis is also the method to prevent blackwater. Such prophylaxis, so far as quinine is concerned, must be properly carried out. Those who take quinine regularly, and in sufficient doses, do not contract blackwater fever. Chill and excesses of all kinds must be avoided. _Treatment._—Much the same treatment should be adopted as that fully described later for malaria. It is especially important to give an aperient at the beginning, and perhaps five grains of calomel is the best form. In every case, no matter how slight, it is essential to ensure, wherever possible, absolute rest in bed and skilled and careful nursing. If it can possibly be avoided a blackwater patient should never be moved from the place where he is taken ill. So long as he has a bed to lie on, a roof to cover him, and some sensible person to look after him who will carry out the doctor’s orders, he should be treated on the spot. It is better, when it can be arranged, for the nurse to go to the patient than for the patient to come to the nurse.