The incidence of acute rejection of the kidney allograft in the world has been around 15% during the period between 2001 and 2003. It is clinically defined as an elevation in the level of serum creatinine by more than 0.3 mg/dL and is diagnosed by kidney biopsy. On pathologic examination, the interstitium of the allograft is diffusely edematous and infiltrated by CD4 and CD8 lymphocytes. Tubulitis occurs when the lymphocytes and monocytes extend into the walls and lumina of the tubules. Presence of leukocytes determines infection or antibody-mediated rejection. Typically C4d staining is negative. Other causes of acute allograft dysfunction included prerenal factors, interstitial nephritis, infection, acute tubular necrosis, toxicity by drugs, and obstruction in the urinary tract. The primary diagnostic assessments include history, especially adherence to immunosuppressive therapy, physical examination, blood and urine laboratory tests, measurement of the serum levels of the drugs, and ultrasonography. Diagnosis of acute cellular rejection depends on biopsy, CD20 staining for refractory cases, negative C4d staining, presence of markers of activating lymphocyte, and proteomic study. Treatment of acute cellular rejection in kidney transplant recipients include pulse steroid for the first rejection episode. It can be repeated for recurrent or resistant rejection. Thymoglobulin and OKT3 are used as the second line of treatment if graft function is deteriorating. Changing the protocol from cyclosporine to tacrolimus or adding mycophenolate mofetil or sirolimus might be effective. Prognosis depends on number of rejection episodes, the use of potent drugs, time of rejection from transplantation, and response to treatment.