Systemic lupus erythematosus (SLE) is an autoimmune disease, involving multiple organs. Diverse manifestations may obscure the diagnosis and confuse our thinking process, especially when few clues are present at the beginning. Serositis is one of the various presentations, and the presence of lupus erythematosus (LE) cell in body fluid may be a hint for the final diagnosis of SLE. Herein, we present a young female patient diagnosed of SLE with initial presentation of lupus peritonitis. Finding of LE cell in ascites prompted us for immunologic survey. Diagnosis of SLE was confirmed with high titer of anti-nuclear antibody and antibody to double-stranded DNA. Cytologic examination of body fluid is mainly useful in detecting malignant cells, but high specificity of this marker aids in early diagnosis of SLE.