BKV is ubiquitous with 80-90% seropositivity. BKV establishes latency in kidney tubular epithelial/urothelial cells but is shed in urine. Reactivation may cause interstitial nephritis (BKV-IN) after renal transplant (Ktxp) and hemorrhagic cystitis (HC) and/or BKV-IN after allo-HSCT. BKV-HC occurs 2-8 wks after HSCT. Diagnosis requires: cystitis, macroscopic hematuria and urine (U) BK VL >7log10 copies/ml. High-level BK viruria occurs in >80% post allo-HSCT but only 5-20% develop HC. Plasma (PL) VLs are elevated in 60% and declining PL-VL correlates with improvement. Re-activation risk factors include young age, male sex; high risk donor; HSCT conditioning with CTX, busulfan, TBI; and grade 2-4 GVHD. Therapy is supportive. There are no FDA approved agents for BKV. Cidofovir (CDV), leflunomide, and FQs have in-vitro BKV activity but clinical use is not supported by rigorous trials. Pooled IVIg contains BKV antibodies. Reduction of immunosuppression improves BKV-IN in Ktxp recipients but is ineffective in allo-HSCT BKV-HC. For >30 years 1st line therapy for malaria has been Artesunate (ART), an artemisinin derivative with a good safety profile. ART has an antiviral spectrum including herpesviruses, HBV, HCV, HIV and BKV. ART inhibits BKV replication in human renal tissue culture via activation of transcription factors NF-κB / Sp-1 and concentration dependent down regulation of phosphorylation by kinase PI3K without cellular cytotoxicity. In-vitro, ATV is synergistic and/or additive with CDV, GCV and Maribavir. Case reports/series of ART use in CMV, HHV6, and resistant HSV-2, are published. CASE: A 22yo M w R/R (see table) T/HRLBCL IIIB underwent a haplo-SCT in PR. At week +8 he developed gross hematuria, cystitis (9-10/10 pain), and urinary outlet obstruction. U-/PL-BK VLs were >100,000,000/ 7.9 mg/dl). PL-BKV VL rose to 196,537copies/ml and both ureters obstructed necessitating bilateral nephrostomy drains. We requested ART via an eIND from the FDA and current US manufacturer. ART load (2.4 mg/kg IV Q 12H x2) followed by 2.4 mg/kg/d administered x 14 d. Cystitis pain fell from 9-10 to 0-2/10 by day 4. U-/PL-BKV VLs fell to 91,980 / There was a strong temporal association between ART treatment and clinical response. CDV and IVIg therapy may also have contributed to the observed improvement. ART therapy may be an emerging option for BKV-IN and HC and further research is warranted.