BackgroundHemophagocytic lymphohistiocytosis (HLH), which is also known as hemophagocytic syndrome (HPS), can be caused by various factors leading to the reactive proliferation of lymphocytes as well as tissues and cells and the release of inflammatory cytokines, which then impair vital organs. Hemophagocytic lymphohistiocytosis, especially malignancy-associated HLH, which is difficult to differentiate from other diseases in the early stage because of its nonspecific and overlapping manifestations, has a high mortality rate once it occurs. Cases of hemophagocytic lymphohistiocytosis triggered by both solid tumors and lymphomas have rarely been reported before. There is still no consensus regarding the first-line treatment for HLH in adults. More attention should be paid to its early diagnosis and treatments.Case presentationA 55-year-old woman who had undergone thyroidectomy in 2014, left mastectomy for left breast cancer in 2013 and twelve rounds of chemotherapy had continuous fever approximately twenty days prior to admission. Tumor cells were found in her right axillary lymph node aspiration smear in the hospital on January 23. The patient was first diagnosed with pneumonia and treated with antibiotics, with little effectiveness. The etiology of fever remained unclear after a series of examinations, which excluded infection and rheumatic diseases and led us to consider noninfectious fever. Continuous hyperpyrexia, pancytopenia and continuous decreases in blood platelets and plasma albumin were suggestive of HLH. Then, bone marrow puncture confirmed this finding and indicated B-cell lymphoma invading the bone. Unexpectedly, during this time, the disease progressed rapidly, culminating in the patient’s death, and no examinations or treatments were performed in time.ConclusionsSecondary hemophagocytic lymphohistiocytosis can be caused by various factors. Malignancy-associated HLH (M-HLH), as a kind of secondary HLH, is difficult to distinguish from primary cancer due to its nonprominent clinical manifestation, thus leading to misdiagnosis and delayed treatment.