Estimation of Chronic rejection in kidney transplant patients of Shahid Labafi-nejad Medical Center from 1984 to 2003 using an eternal Weibull regression: A historical cohort study B. Golestan, SM Hosseini Moghaddam, M Nafar, K Rennolls, K Mohammad Urology Nephrology Research Center. Shaheed Beheshti University of Medical Sciences, Tehran University of Medical Sciences, Tehran, Iran We estimated the chronic rejection in kidney transplant patients using an eternal Weibull regression. In this historical cohort study, we enrolled all patients with chronic renal failure admitted in Shahid Labafi-nejad center from 1984 to 2003. The basic assumption regarding survival analysis is that given enough time all cases will experience the event of interest; however, there are situations like renal transplantation where this assumption is not necessarily met and hopefully a considerable percentage of cases will never experience chronic rejection and remain eternal according to this event. As a result, the survival function needs to be modified according to this eternal proportion. We used Weibull distribution and estimated the survival function in the unmodified and modified forms. We also applied the modification in the regression form. Using Mat. Lab 7.0 to estimate the model parameters, we considered the eternal proportias a logistic-type function of the covariates. The chance of chronic rejection was almost 2-fold among those who received kidney transplant before 1996. (exp (0.67=1.95). Males had a chance of rejection 20% less than females (exp-0.23). Considering the eternity, Weibull model was again fitted to cases who received renal transplants after 1996. Using mycophenolate mofetile, treatment protocol was changed after 1996 expecting less chronic rejection; thereafter, the eternal proportion was estimated to be 0.81. This seems quite considerable as a percentage of non failure cases. Providing a non zero eternal proportion, the modified model will be superior to the unmodified model.