A study to determine the reproducibility of histopathological findings and diagnoses of rejection was carried out on a series of 42 liver allograft needle biopsy specimens by five pathologists practicing at four liver transplant centers. Pathologists from each of the four centers read each slide independently on two different occasions and were asked to assess 12 histopathological features and render a diagnosis. For all histological variables, the intrarater agreement was higher than the interrater agreement. Moderate to excellent agreement occurred among the pathologists about all histological variables thought to be important in establishing the diagnosis of acute rejection (i.e., portal tract inflammation, subendothelial inflammation and bile duct damage). Other variables such as lobular disarray, bile duct proliferation and particularly arteritis, however, were only fairly or poorly reproducible. Surprisingly, the diagnosis of acute rejection was more reproducible than the individual histopathological findings that were thought to be the basis for the diagnosis. The agreement for the diagnosis of chronic rejection, however, varied according to observer. We noted that relatively inexperienced observers within this group had some difficulties agreeing with more experienced observers in establishing a diagnosis of chronic rejection. These findings demonstrate that the histopathological diagnosis of acute cellular liver allograft rejection is highly reproducible within a group of experienced pathologists and that this diagnosis can be pooled in a common data base with confidence. (HEPATOLOGY 1991;14:751–755).