Macrophage activation syndrome (MAS) is a rare, but potentially life-threatening complication of systemic lupus erythematosus (SLE). A bone marrow biopsy often provides pathologic evidence of MAS, and MAS usually responds to corticosteroids alone or with the addition of cyclosporine A. We describe a case of MAS developing in a pregnant patient with SLE, who presented with fever and pancytopenia. Extensive investigations could not find the evidence of infection. Although intensive medical treatment was performed with a suspicion of MAS based on clinical grounds, no response was observed and bone marrow biopsy showed no evidence of hemophagocytosis. Positron emission tomography/computed tomography (PET/CT) suggested the possible cause of fever was in the spleen where fluorodeoxyglucose uptake was markedly increased. After splenectomy, the patient was improved and numerous hemophagocytic macrophages were proved in the splenic tissue. With this unique case, we would like to emphasize that bone marrow biopsy cannot always be relied on in making a diagnosis of MAS and PET/CT can provide helpful information in the diagnosis of MAS.