Few studies have evaluated the impact of viral infections on the daily management of patients with systemic lupus erythematosus (SLE). We analyzed the etiology and clinical features of acute viral infections arising in patients with SLE and their influence on the diagnosis, prognosis, and treatment of SLE. Cases occurring within the last 5 years were selected from the databases of 3 large teaching hospitals. Acute viral infections were confirmed by the identification of specific antiviral IgM antibodies and subsequent seroconversion with detection of specific IgG antibodies. In autopsy studies, macroscopic findings suggestive of viral infection were confirmed by direct identification of the virus or viruses in tissue samples. We performed a MEDLINE search for additional cases reported between January 1985 and March 2008. We included 88 cases (23 from our clinics and 65 from the literature review) of acute viral infections in patients with SLE. Twenty-five patients were diagnosed with new-onset SLE (fulfillment of the 1997 SLE criteria) associated with infection by human parvovirus B19 (n = 15), cytomegalovirus (CMV; n = 6), Epstein-Barr virus (EBV; n = 3), and hepatitis A virus (n = 1). The remaining 63 cases of acute viral infections arose in patients already diagnosed with SLE: in 18 patients, symptoms related to infection mimicked a lupus flare, 36 patients, including 1 patient from the former group who presented with both conditions, presented organ-specific viral infections (mainly pneumonitis, colitis, retinitis, and hepatitis), and 10 patients presented a severe, multiorgan process similar to that described in catastrophic antiphospholipid syndrome-the final diagnosis was hemophagocytic syndrome in 5 cases and disseminated viral infection in 5. Twelve patients died due to infection caused by CMV (n = 5), herpes simplex virus (n = 4), EBV (n = 2), and varicella zoster virus (n = 1). Autopsies were performed in 9 patients and disclosed disseminated herpetic infection in 6 patients (caused by herpes simplex in 4 cases, varicella in 1, and CMV in 1) and hemophagocytic syndrome in 3. A higher frequency of renal failure (54% vs. 19%, p = 0.024), antiphospholipid syndrome (33% vs. 6%, p = 0.023), treatment with cyclophosphamide (82% vs. 37%, p = 0.008), and multisystemic involvement at presentation (58% vs. 8%, p < 0.001); and a lower frequency of antiviral therapy (18% vs. 76%, p < 0.001) were found in patients who died, compared with survivors. The most common viral infections in patients with SLE are parvovirus B19 (predominantly mimicking SLE presentation) and CMV (predominantly presenting in severely immunosuppressed patients). CMV infection may mimic a lupus flare or present with specific organ involvement such as gastrointestinal bleeding or pulmonary infiltrates. Other herpesviruses are common in immunosuppressed SLE patients and may produce a wide range of manifestations. Physicians should examine the pharynx, eyes, skin, and genitalia and should conduct serologic and molecular studies to improve early detection of viral infection in patients with SLE.