To evaluate serum levels of soluble CD30 (sCD30) for prediction and diagnosis of acute kidney allograft rejection.We prospectively measured serum levels of sCD30 before kidney transplantation, 5 days postoperatively, and at creatinine elevation episodes. The predictive value of sCD30 for diagnosis of AR within the 6 postoperative months was assessed in 203 kidney recipients from living donors. Serum levels of the pretransplant and postoperative sCD30s were 58.10 ± 52.55 mg/dL and 51.55 ± 49.65 mg/dL, respectively (P = .12). Twenty-three patients experienced biopsy-proven acute rejection, 28 had acute allograft dysfunction due to non-immunologic diseases, and the remaining 152 had normal creatinine levels. Pretransplant sCD30 was not different between patients with and without AR (P = .77). Post transplant sCD30 was higher in 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 matched for age, sex, and donor source (P < .001). The post transplant sCD30 levels allowed a differentiation of kidney recipients with subsequent AR within 6 postoperative months from those without AR (cutoff value, 41 U/mL; sensitivity; 70%; specificity, 71.7%). Graft survival rates were not associated with sCD30 values. The level of sCD30 at creatinine elevation was not associated with AR diagnosis. Post transplant sCD30 level is higher in patients with acute rejection, but its clinical value requires further investigation. To elucidate the diagnostic value of sCD30, we have to better understand factors that influence this marker.