Understanding of the events preceding acute cellular rejection of kidney transplants would be useful in the development of immunosuppressive strategies to prevent rejection. Information about these events in humans has been scarce, because of the lack of early, serial, biopsy samples. We took daily fine needle aspirates from kidney allografts for the first 10 days after transplant. Samples were analyzed by morphological cytology of graft-infiltrating cells, and reverse transcriptase-polymerase chain reaction for detection of interleukin (IL)-2, IL-4, IL-6, IL-10, and gamma-interferon gene expression. During the first 4 days, all of the grafts developed a low-grade monocyte-rich mononuclear cell infiltrate, accompanied by IL-10 gene expression. Thereafter, the infiltrates either remained stable or intensified. Of the 13 grafts with dense infiltrates, seven developed graft dysfunction. The remaining six did not, despite significant interstitial infiltrates. Both rejecting and nonrejecting dense infiltrates were associated with a biphasic pattern of IL-2 and gamma-interferon gene expression, preceding and accompanying lymphocytic graft infiltration. Grafts that did not develop dense infiltrates had no detectable IL-2 or gamma-interferon gene expression and did not suffer cellular rejection during the study period. The development of both rejecting and nonrejecting infiltrates was strongly associated with DR mismatches between donor and recipient. IL-2 and gamma-interferon gene expression are necessary, but not sufficient, for the development of acute cellular rejection in the first 10 days of kidney transplantation, and are more closely associated with the period leading up to rejection than with the period of graft dysfunction.