Serum levels of soluble CD30 (sCD30) have been considered as a predictor of acute kidney allograft rejection. We have evaluated the pre-transplant and post-transplant levels of sCD30 with the aim of determining its value in predicting and diagnosing kidney rejection. We measured sCD30 serum levels before kidney transplantation, 5 days post-operatively, and at creatinine elevation episodes. The predictive value of sCD30 for diagnosing acute rejection (AR) within the first 6 post-operative months was assessed in 203 kidney recipients from living donors. Pre-transplant and post-operative levels of serum sCD30 were 58.10 +/- 52.55 and 51.55 +/- 49.65 U/ml, respectively (P = 0.12). Twenty-three patients experienced biopsy-proven acute rejection, and 28 had acute allograft dysfunction due to non-immunologic diseases. The pre-transplant sCD30 level was not different between patients with and without AR. However, post-transplant sCD30 was higher in the AR group. The median serum level of post-transplant sCD30 was 52 U/ml in the AR group and 26.3 U/ml in a control group (P < 0.001). The relative changes of sCD30 on day 5 were higher in patients with AR (P = 0.003). Based on post-transplant sCD30 levels, we were able to differentiate between kidney recipients who experienced an AR within 6 months post-surgery and those without an AR (cutoff value 41 U/ml; sensitivity 70%; specificity 71.7%). The level of sCD30 during periods of elevated serum creatinine was not independently associated with the diagnosis of AR. Post-transplant sCD30 levels and their relative changes are higher in patients experiencing AR. We propose further studies on the post-transplant trend of this marker for the prediction of AR.