The patient, a 65-year-old man, presented with swelling and pain in his right scrotum three months ago. He was first admitted to our hospital 5 years ago with a cough and fever. The CT scan of the abdomen showed multiple soft tissue densities behind the peritoneum and the serum IgG4 3200 mg/dL. The patient was definitively diagnosed with IgG4-RD as well as secondary retroperitoneal fibrosis by the gold standard of histopathology. He was in long-term remission after regular treatment with mycophenolate mofetil (1g Bid) and prednisone (0.6 mg/kg), followed by six months, four years, and a recent abdominal CT scan showed no progression of retroperitoneal fibrosis. He had no previous history of pulmonary or urinary tuberculosis and no significant family history. On examination, a mobile mass of approximately 2 cm in diameter was found in the right scrotum. The mass was hard and tender. No other positive signs were found. Laboratory tests showed erythrocyte sedimentation rate 69 mm/h (reference range 0-15 mm/h), C-reactive protein 29.9 mg/L (reference range 0-10 mg/L), complement C3 0.58 g/L (reference range 0.9-1.8 g/L), complement C4 0.03 g/L (reference range 0.1-0.4 g/L). Serological examination showed elevated IgG4 levels to 2070 mg/dL (reference range 6-130 mg/dL). Negative for prostate-specific antigen (PSA), tumor markers, and tuberculosis antibodies. Scrotal ultrasound demonstrating enlargement of the right testicle. Enhanced computed tomography of the scrotum showed a nodular hyperdense shadow with a diameter of approximately 23 mm on the right epididymis, with significant heterogeneous enhancement on the enhanced scan (Figure 1). After ruling out testicular tuberculosis and hydrocele due to retroperitoneal fibrosis secondary to IgG4-RD, given the potential risk of malignancy, he underwent a pathological biopsy by puncture of the right epididymis. Pathological biopsy of the right epididymis showed infiltration of plasma cells, lymphocytes, and a few neutrophils with occlusive vasculitis changes in the tissue (Figure 2A). IgG4+ plasma cells stained positive, with an IgG4/IgG ratio of more than 40% and more than 30 IgG4+ plasma cells per high-power field (Figure 2B). A diagnosis of IgG4-RD involving the testicles was made. Prednisone 30 mg/d was given for three weeks. No scrotum swelling or pain was observed at the follow-up after six months. The timeline of diagnosis, treatment, and prognosis for this case is shown in Figure 3.