<p>Concurrent anti-neutrophil cytoplasmic antibody (ANCA)-associated nephritis and immunoglobulin G4 (IgG4)-related nephritis is a rare clinical entity with incompletely understood pathogenesis and progression. We describe the case of a 56-year-old woman, without significant past medical history, who presented with a recurrent low-grade fever for over 3&#xa0;months, occasional non-productive cough, and acute renal insufficiency, but without chills. Empirical anti-infective therapy was ineffective. Laboratory evaluation revealed moderate anemia and microscopic hematuria without proteinuria on urinalysis. Immunological testing was positive for anti-myeloperoxidase (MPO) antibodies, supporting a preliminary diagnosis of ANCA-associated vasculitis with renal involvement. Renal biopsy demonstrated crescentic glomerulonephritis coexisting with tubulointerstitial nephritis (TIN), with predominant infiltration by IgG4-producing plasma cells, despite a normal serum IgG4 concentration. Following confirmation of no therapeutic contraindications, the patient received corticosteroids in combination with rituximab. At a 2-week follow-up, renal function had improved, erythrocyte sedimentation rate and C-reactive protein were normalized, and urinalysis was negative for occult blood and protein, indicating effective disease control. Cases with concurrent ANCA-associated vasculitis and IgG4-related renal disease are exceedingly uncommon. Their clinical manifestations are often atypical, and routine laboratory assessments are insufficient for differentiation, typically requiring integrated histopathological analysis for definitive diagnosis. Further clinical observations and laboratory studies are needed to clarify the pathogenesis, improve diagnostic accuracy, and establish optimal treatment strategies for this complex condition.</p>