Introduction:
Systemic lupus erythematosus (SLE) is a chronic, multi-organ inflammatory disease of autoimmune origin. In some cases, SLE may be the first symptom of cancer. Paraneoplastic syndromes in rheumatology manifest as symptoms of inflammation of joints, muscles, and blood vessels caused by cancer, but not directly related to tumour invasion or the presence of metastases. Systemic lupus erythematosus as a paraneoplastic syndrome was observed in patients with lymphoma, breast cancer, lung cancer, and, rarely, gastrointestinal cancer.

Case description:
We present the case of a 69-year-old man who was admitted to the rheumatology clinic in September 2025 with an inflammatory rash on his face, neck, upper limbs and back, as well as oral ulcers, arthritis and weight loss. Laboratory tests revealed pancytopenia, low complement C3 and C4, positive antinuclear antibodies, and positive anti-ribosomal protein P. The patient fulfilled the 2019 European Alliance of Associations for Rheumatology/American College of Rheumatology classification criteria for SLE. Thoracic, abdominal and pelvic computed tomography scans showed pleural effusion, a single enlarged mediastinal lymph node, irregular thickening of the splenic flexure of the colon, and splenomegaly. Colonoscopy revealed a polypoid lesion in the distal transverse colon and descending colon, measuring approximately 3 cm; histopathological examination showed a tubular adenoma of the large intestine with high-grade dysplasia and a focus of invasion (adenocarcinoma not otherwise specified [NOS], low grade). The treatment consisted of glucocorticosteroids in high doses and hydroxychloroquine. As a result of the treatment, the skin and mucosal lesions, as well as arthritis, significantly decreased. In December 2025, a partial colon resection was performed. Since the operation, there have been no signs of SLE activity.

Conclusions:
Any systemic connective tissue disease, including SLE, can be a paraneoplastic syndrome. Particular attention should be given to the possibility of cancer in cases involving severe skin lesions, general symptoms, and older age at the onset.