Infections are an important cause of morbidity and mortality in systemic lupus erythematosus. We aimed to determine the incidence and characteristics of infections in patients hospitalized because of systemic lupus erythematosus, and to identify which factors influence their outcome. The medical records of patients with systemic lupus erythematosus hospitalized between January 2002 and December 2007 were reviewed according to a standardized case form including demographic, clinical, and therapeutic data. The diagnosis of infection was based on clinical findings, the identification of the causative agent or response to antibiotic treatment. The study included 473 patients (mean age 30 +/- 11 years; 421 (89%) female) who were hospitalized for a mean of 13 +/- 9 days. A community-based infection was suspected in 268 (57%) at admission; the diagnosis was confirmed in 96 patients (22%) and ruled out in 20 (4.2%); nevertheless, 152 patients (32%) received antibiotics on an empirical basis. A nosocomial infection was suspected in 63 (13.3%) of 453 patients and was confirmed in 59 (12.5%). The two most common community-acquired and nosocomial infections affected the respiratory and genitourinary tracts. Gram-negative bacteria were major etiological agents isolated. In the multivariate analysis, community-based infections associated with mucocutaneous, renal, or central nervous system disease activity as well as fever, and Mex-SLEDAI at admission and nosocomial infections to azathioprine use, infection at admission, disease duration, and hospitalization >7 days. We conclude that infections are an important cause of hospitalization of systemic lupus erythematosus patients. Risk factors include disease activity, use of immunosuppressants, disease duration, and length of hospital stay.