After an incubation period of from 5 to 7 days the disease sets in abruptly with rigors, headache, muscular pains and vomiting. The patient is prostrated and has the appearance of extreme illness; the face flushed and the conjunctivae injected. There is fever of an irregular type usually running between 102° to 104°F. for the first three or four days, when it begins to fall by lysis although occasionally by crisis about the fifth day. Following a few days of moderate fever or normal temperature there is a tendency for a second rise toward the end of the second week, which continues for approximately another week when a slow convalescence sets in in favorable cases. The secondary fever shows rather marked oscillations. Jaundice appears about the third or fourth day with marked tenderness of the liver and slight or moderate enlargement of the spleen. The urine is scanty and high colored showing albumin and bile pigments. Early in the second week urine of a low specific gravity is excreted in large amounts. The pulse is rapid at first to become slow with the appearance of the jaundice. There is a tendency to sleeplessness and nocturnal delirium and in unfavorable cases the “typhoid state” comes on. Pains in the nape of the neck and calf muscles are common features. Haemorrhages, starting as epistaxis, are commonly observed. Next to epistaxis intestinal haemorrhage is the most frequent. Haematuria is rarely observed. The red cells and the haemoglobin fall in percentage with an increase in the polymorphonuclears. The leucocytosis runs about 15,000. The spirochaetes may be found with the dark-field illumination early in the disease in a blood examination and later on in the urine. DIAGNOSIS It must be remembered that the causative spirochaete is very scarce in the blood of this disease which is usually not the case with relapsing fever showing jaundice. It is best to inject 2 to 4 cc. of blood into young guinea pigs intraperitoneally and after the development of the infection in the animal we find the spirochaetes rather abundantly in preparations made from liver emulsions and examined with dark-field illumination. Noguchi cultured the organism from the blood early in the disease but found it difficult. Wataguchi states that when the blood is simply diluted with water and kept at a temperature of about 25°C. growth may be obtained by the fourth day, reaching a maximum by the second week. Agglutinins appear in the blood about the end of the first week and cultures may be agglutinated by such serum diluted as high as 1 to 500. Bilious remittent fever shows earlier jaundice, a more rapid pulse rate and malarial parasites. In yellow fever there is more marked rachialgia and earlier and more marked albuminuria. The marked leucocytosis of Weil’s disease should be of differentiating value. The early jaundice and haemoglobinuria of blackwater fever should distinguish this disease.