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Key Clinical Message IgG4‐related disease is a rare and emerging pathology, characterized by the appearance of pseudotumors. Due to the ability to mimic other pathologies, it is essential to consider it as a differential diagnosis in multisystemic processes. The diagnosis is challenging, requiring a multidisciplinary approach, to minimize the associated morbidity and mortality. Abstract IgG4‐related disease (IgG4‐RD) is a rare, emerging, systemic and chronic pathology, characterized by the appearance of pseudotumors resulting from tissue infiltration by IgG4‐positive plasma cells that promote eosinophilic inflammation of the tissue with subsequent fibrosis. We present the case of a male, 45‐year‐old patient, with marked weight loss and skin pallor detected by his family doctor during a child health consultation of his daughter. When questioned, the patient referred complaints of postprandial discomfort in the left hypochondrium with a feeling of fullness, weight loss, chronic fatigue and hyperhidrosis that had lasted for a month. On physical examination, he was pale, and had pain at palpation of the left hypochondrium. Laboratory data showed increased inflammation markers, abdominal ultrasound and CT demonstrated numerous enlarged lymph nodes in the upper quadrants, raising concern for a malignant lymphoproliferative process. Serological, imaging, clinical and laparoscopic excisional biopsy revealed features of IgG4‐related disease and excluded malignant lymphoproliferative disease. The immediate response to treatment with oral prednisolone 30 mg/day also contributed for diagnosis confirmation. Due to refractory disease after gradual prednisolone reduction, second‐line therapy with rituximab was initiated. Over the 6 years of follow‐up, the patient presented multiple exacerbations characterized by the emergence of systemic symptoms, being maintained under close clinical and imaging follow‐up by reumathology, infectious diseases, and family medicine specialists.