CASE PRESENTATION A female in her 30s developed renal failure secondary to mixed connective tissue disease-associated immune complex glomerulonephritis. Her past medical and family history was otherwise unremarkable. She underwent a pre-emptive renal transplantation from her mother, receiving induction with dacluzimab and had an uneventful post-transplant course. Her immunosuppressive regimen at discharge included prednisone 5 mg daily, mycophenolate mofetil 1000 mg twice daily, tacrolimus 4 mg twice daily, and prophylaxis with oral gancyclovir and trimethoprim–sulfamethoxazole, atenolol for hypertension, and famotidine, iron and calcium/vitamin D supplements. Her baseline serum creatinine after the transplant was 1.5 mg/dl. Two years after receiving the transplant, she developed fever and dysuria and was treated for a presumed urinary tract infection (negative urine cultures). Her serum creatinine during this episode increased to 1.9 mg/dl and remained elevated in this range. Her physical examination was unremarkable and her blood pressure was 130/80 mm Hg. A renal ultrasound did not show any abnormalities. A renal biopsy was performed. RENAL BIOPSY FINDINGS Sampling for light microscopy consisted of a single core of renal cortex containing eight glomeruli, all of which appeared histologically unremarkable. There was diffuse, moderate to severe interstitial inflammation involving the entire cortex sampled, and composed of lymphocytes and less prominent plasma cells. The interstitial inflammation extended across tubular basement membranes to produce extensive tubulitis with multiple tubules containing greater than 10 lymphocytes. Proximal tubules also displayed diffuse degenerative changes and interspersed, enlarged, hyperchromatic nuclei with intranuclear viral inclusions typical of BK virus. This diagnosis was subsequently confirmed by immunohistochemical staining for SV40. There was moderate tubular atrophy and interstitial fibrosis involving 50% of the cortex sampled. Blood vessels appear unremarkable (Figure 1). The renal biopsy findings were diagnostic of BK virusassociated interstitial nephritis (BKVIN). In the light of the diffuse distribution of the interstitial inflammation and tubulitis but more localized viral inclusions, the possibility of coexistent acute cellular rejection could not be excluded.