BACKGROUND A flare, or flare-up, of systematic lupus erythematosus (SLE) is diagnosed by an increase in disease activity in one or more organs, new symptoms, or changes in laboratory measurements. A hematoma can occur in the sheath of the rectus abdominis following muscle trauma or rupture of an epigastric vessel, or it can occur spontaneously. This report is of a 28-year-old woman who presented with a clinical flare of SLE and abdominal pain due to rectus sheath hematoma. CASE REPORT A 28-year-old woman had been suspected of having SLE 9 years ago and had received glucocorticoid therapy combined with hydroxychloroquine. However, lupus flared after she discontinued glucocorticoids, and she was admitted with a 1-month history of marked generalized edema, abdominal distension, frothy urine, and massive ascites. During hospitalization, she abruptly developed a continuous, stabbing abdominal pain and a bulge over the right abdomen as a result of straining during a bowel movement. On examination, a well-demarcated round mass that measured 121 mm × 96 mm was detected in the right quadrant. Abdominal emergency computed tomography revealed a right rectus sheath hematoma (21.4×4.7 cm). After her condition improved, the patient underwent an ultrasound-guided renal biopsy and was diagnosed with class III (A/C) and class V lupus nephritis. CONCLUSIONS This case has shown that spontaneous rectus sheath hematoma can occur without a history of trauma in a patient with an exacerbation of SLE. This association appears to be rare, and the cause is unknown.