Damage to the renal cortical microcirculation, an early event in the course of acute rejection crisis (ARC), usually precedes measurable functional derangements in the transplanted kidney. Direct assessment of cortical blood flow by radionuclide renography may provide a sensitive and reliable index to the diagnosis of ARC, with particular regard to the differential diagnosis of ARC and ATN. Computer generated time-activity curves of global, cortical, and medullary renal blood flow were analyzed in 67 instances (35 patients) of renal allograft dysfunction and correlated with needle biopsy of these kidneys. No increase in cortex perfusion index (CPI), i.e., decrease in cortical perfusion, was found when the patients were suffering from ureteral obstruction or drug and viral nephropathy (mean perfusion index (PI) increase (8%). In contrast, a marked increase in CPI of 193% was noted in ARC. Global and medullary PI increased only 116%. As a result, global and medullary PI were capable of diagnosing ARC in only 73% and 55% of the cases, respectively, whereby cortex PI correctly diagnosed ARC in 94% of the cases. Selective analysis of cortical perfusion may thus enhance the accuracy of [99mTc]DTPA scans (radionuclide renograph) for the early detection of ARC and in differentiating ARC from nonimmunological causes of kidney allograft dysfunction.