Fine-needle aspiration biopsies were used for clinical monitoring of liver allografts; 21 patients with an inflammatory episode of acute rejection (12.0 +/- 3.3 CIU at the peak) were studied; all episodes were reversible. The inflammatory infiltrate consisted mainly of lymphoid cells, including lymphoid blasts, with increased numbers of class II and IL-2-receptor expressing lymphocytes. No lymphoid activation was seen in corresponding blood specimens. A rapid response to antirejection therapy with high-dose steroids was recorded by FNAB. Another group of 7 recipients without rejection developed a severe CMV disease. The CMV disease was associated with mild inflammation in the FNAB (3.2 +/- 0.9 CIU at the peak), but fewer blast cells and activated cell types were recorded in the graft, and blast cells and lymphocyte activation were seen in the blood specimens, as well. The inflammation disappeared from the FNABs during successful antiviral treatment. The cellular findings of rejection and CMV infection were significantly different, and the presence or absence of rejection could be firmly established by FNAB.