Abstract: Objective To improve our understanding of chronic myelomonocytic leukemia(CMML) in etiological diagnosis of fever of unknown origin(FUO) and to study the clinical characteristics of CMML and the relation between Sweet's syndrome(SS) and CMML. Methods Clinical data about 1 patient with FUO accompanying SS were retrospectively analyzed with its relevant literature reviewed. Results The patient suffered from chronic fever with painful skin rashes. Laboratory test showed that the number of peripheral mononuclear cells was>1.0×10⁹/L and bone marrow morphology revealed pathological hematopoietic cells. The patient who was diagnosed with SS accompanying pneumonia and cytomegalovirus infection had no response to anti-infection and hormone therapy. After other causes that increased the number of mononuclear cells were excluded, the patient was finally diagnosed with CMML. Conclusion CMML is manifested as chronic fever with SS as its precursory symptom.