Systemic lupus erythematosus (SLE) is a serious multisystem disease that has a striking propensity to affect women. The cause of SLE remains elusive. Fetomaternal cell trafficking, or the passage of fetal cells into the maternal circulation, is now a well-established phenomenon. In addition, fetal cells have been implicated in the development of preeclampsia and in the pathogenesis of scleroderma. We undertook this study to determine whether fetomaternal cell trafficking might also be involved in pathogenic processes in SLE. Fluorescence in situ hybridization analysis was performed using X and Y chromosome-specific probes on affected and unaffected tissue obtained at autopsy from a woman who had previously given birth to 2 males and who had died of complications of SLE. The goal of the analysis was to detect the presence of male cells of putative fetal origin. Male cells were found in every histologically abnormal tissue type that was examined, but were not found in histologically normal tissue. These data suggest that fetal cells may be associated with SLE. It is unclear whether their presence may be related to disease causation, an effect of disease progression, or unrelated to disease pathology. However, this case study is an important step toward understanding the potential relationship between fetomaternal cell trafficking and SLE pathology.