Although renal transplantation extends life expectation in older age groups, some studies reported high number of deaths with functioning grafts in elderly recipients. This maybe due to different complications in this group of patients. This study compares the causes of post renal transplantation re-hospitalizations in elderly with young patients. We randomly selected 567 records of rehospitalization after kidney transplantation from 1994 to 2006 in a university-based referral center for kidney transplantation hospital, Tehran, Iran. According to the Age of Recipients at the Time of Transplantation (ARTT), hospitalizations were divided into Group I (ARTT<=50, n=185) and Group II (ARTT >50, n=382). The causes of re-hospitalization were categorized into infection, surgical complication, macrovascular disease, malignancy, renal dysfunction and nephrolithiasis. The frequency of re-hospitalization causes were compared in 2 groups. In comparison to younger patients, a higher rate of admissions due to infection (52.4% vs. 42.9%, p=0.033), surgical complications (9.7% vs. 4.7%, p=0.022), macrovascular diseases (3.8% vs. 1.0%, p=0.027), and a lower rate due to renal dysfunctions (33.5% vs.51.6%, p=0.00) were seen. No significant difference was found in the frequency of admissions because of malignancies (1.1% vs. 1.6%, p>0.05), or nephrolithiasis (1.1% vs. 3.7%, p>0.05) between the two groups. According to the results of this study, the causes of re-hospitalization after kidney transplantation are mainly different in older patients. This implies a specific attention to older age groups after transplantation and an age - specific follow up strategy. The health system also needs to be planned appropriately.