Between May 1981 and April 1986, 1402 patients were admitted to the Department of Primary Care Medicine of Kawasaki Medical School Hospital. Of these, 452 patients had a slight fever ranging from 37.0°C to 37.9°C. We analyzed those patients clinically. Infection ranked first as the cause of slight fever, followed by malignancy, collagen disease and others. About 50% of the cases were of unknown origin, and many cases with CRP and ESR almost within the normal range convalesced satisfactorily. The measurement of CRP and ESR in slight fever patients were useful to exclude organic slight fever. The cases with a slight fever of unknown origin appearing for a long term also often had nonorganic diseases such as depression or neurosis. Almost all of these cases were placed in the category of habitual hyperthermia
