As a consequence of more intensified immunosuppression, post-transplant lymphoproliferative disease (PTLD) is increasingly observed in patients after solid-organ transplantation. Beta2-microglobulin, a low-molecular weight protein (MW 11.8 kDa), is produced by all nucleated cells as part of the HLA complex. Its serum concentration is directly correlated with prognosis in patients with lymphatic neoplasms. Like other low-molecular weight proteins, beta2-microglobulin is eliminated by glomerular filtration. This complicates its use as a tumor marker in renal insufficiency. Cystatin C, a low-molecular weight protein of 13.3 kDa, is a new marker of kidney function largely unaffected by extrarenal disease. We, therefore, sought to assess the potential of the beta2-microglobulin/cystatin C ratio (beta2M/Cys) as a marker of lymphoproliferation.