Routine transplant aspiration cytology (TAC) after liver transplantation gives detailed information that concerns immunologic events in the graft. TAC can be helpful for diagnosis of acute rejection, but it also detects morphological signs of rejection without clinical correlate ("subclinical rejection"). The aim of this study was to systematically evaluate factors that influence the development of early clinical and subclinical rejection and to analyze the relevance of these early immunologic processes for the long-term course. The study includes the course of 340 patients after liver transplantation between 1988 and 1995 in whom TAC was performed routinely and who were followed for a minimum of 3 years. TAC findings were correlated with the following various clinical parameters: (1) Overall early clinical rejection occurred in 17.4%, subclinical rejection in 59.1%, and no immune activation was seen in 23.5% of patients. (2) Incidence of early clinical and subclinical rejection was markedly influenced by type of immunosuppression. (3) Basic disease and extent of preservation injury had only a minor influence; there was a trend towards lower early rejection associated with more severe preservation damage, increased patient age, and early retransplantation. (4) Presence of early clinical or subclinical rejection was not associated with a higher incidence of chronic dysfunction. (5) Falsely indicated antirejection treatment was associated with inferior graft survival. Subclinical rejection is very frequent early after liver transplantation, requires no treatment, and has no long-term adverse effect. Incidence of early clinical rejection is mainly determined by initial immunosuppression; its occurrence has no negative long-term effects and may even be associated with a lower risk for later immunological complications. Thus, the incidence of early acute rejection is no adequate parameter for evaluating the quality of an immunosuppressive treatment protocol.