In clinical practice, we sometimes encounter a situation where a patient has a long-standing fever that has not been treated with repeated anti-infection therapy, and the common indicators of infection, autoimmune disease, and tumor are all negative, and no abnormalities are seen in routine imaging tests such as lung CT and abdominal ultrasound. The patient has no symptoms other than fever, and the doctor’s examination does not show any obvious positive signs. This is often a confusing time. Here, I have to draw attention to the possibility that the patient has the presence of lymphoma. A significant number of patients with lymphoma are among the most difficult to diagnose among febrile illnesses, and in some cases it is not even clear until autopsy. Lymphoma does not have to be a swollen lymph node on the surface of the body as many people think, and studies have shown that only 40% of lymphomas have swollen surface lymph nodes as their first symptom. So how do you go about considering the possibility of lymphoma? In fact, there are sometimes clues for us to look for: 1) clinically unexplained anemia and accelerated blood sedimentation; 2) inexplicable elevation of blood β2 microglobulin and lactate dehydrogenase; 3) night sweats and weight loss in addition to fever; 4) enlarged spleen on physical examination, ultrasound or CT. If these clues are present, further imaging of the whole body should be performed, including CT of the head, sinuses, chest, abdomen and pelvis, and whole body isotope bone scan. A whole-body PETct examination is preferable when available. These imaging examinations are likely to find lymphoma lesions for us, and pathological diagnosis after biopsy of the lesions is a necessary tool to confirm the diagnosis of lymphoma. In particular, PETct has an important position in the diagnosis of lymphoma. Some studies have found that the sensitivity of PETct in detecting lymphoma in unexplained fever is as high as 100% and the specificity is as high as 96%, which means that PETct can detect most of the lymphomas in unexplained fever, and only 4% of the patients it thinks are lymphoma are misidentified, so it is a powerful weapon in the diagnosis of lymphoma at present. a powerful weapon in the diagnosis of lymphoma.