AIMS:Renal allograft biopsies play a critical role in renal transplantation. Acute rejection characterized by tubulitis and intimitis is of primary concern. There is an association between eosinophilic infiltrates and irreversible acute rejection; however, the significance of eosinophils in biopsies that fall short of the diagnostic threshold for acute rejection has not been well studied. This report describes clinical course, treatment and long-term outcome of 5 transplant recipients with biopsy histology that showed borderline changes associated with eosinophilic infiltrates. METHODS:Clinical records were selected for review on the basis of biopsy histology satisfying the following criteria: presence of interstitial infiltrates with eosinophils, absence of definitive criteria for acute rejection and absence of findings suggestive of infection or cyclosporine toxicity. RESULTS:All identified biopsies occurred within the first month of transplantation, and histology showed varying degrees of patchy mononuclear cell infiltrates composed of lymphocytes, with eosinophilic infiltrates, but no evidence of acute rejection based on Banff criteria. These patients were taking trimethoprim-sulfamethoxazole and ranitidine at the time of biopsy. Serum creatinine returned to baseline levels in each case after stopping both drugs, and remained stable during the duration of follow-up without any documented episode of acute rejection. No patient received specific therapy for acute rejection. CONCLUSION:This report suggests that independent of decisions on treatment with high-dose steroids or anti-lymphocyte antibody preparations, the management algorithm should include stopping drugs associated with acute interstitial nephritis when non-diagnostic biopsies show eosinophilic infiltrates.