Given its association with Epstein-Barr virus (EBV), there is considerable interest in assessing the impact of prophylactic anti-viral therapy on post-transplant lymphoproliferative disorder (PTLD). A recently completed multi center case-control study assessed the impact of immunosuppressive therapy on PTLD risk among renal transplant patients and collected information on the use of anti-viral therapy. Biopsy-confirmed PTLD cases (n = 100) were matched to 375 controls by center, date of transplant, and age. Data were collected on immunosuppression and rejection therapies, demographics, pre-transplant viral status, number of rejections, and anti-viral use. With adjustment for known risk factors, prophylactic anti-viral use was associated with up to 83% reduction in the risk of PTLD, depending on the anti-viral agent. These results were stronger for the first year post-transplant. For every 30 days of ganciclovir treatment, risk of PTLD during the first year was lower by 38% (Odds Ratio [OR]= 0.62; 95% confidence interval [CI]= 0.38-1.0); acyclovir effects were less striking (OR = 0.83; 95% CI = 0.59-1.16). Anti-viral therapy appears to play a role in reducing the risk of PTLD in renal transplant patients. Ganciclovir may be more potent than acyclovir.