We measured the serum levels of p53 mutant protein (p53M-ELISA) in 65 patients with plasma cell dyscrasia (PCD) and compared them with some conventional laboratory variables. Our aim was to assess, for the first time, the potential of this parameter as a new marker for laboratory management of PCD. Twenthy-tree out of 65 patients had monoclonal gammapathy of undetermined significance (MGUS) and 42 suffered from multiple myeloma (MM). MM patients, with no prior chemotherapy consecutively entered this study. They were treated with standard regimens of Melphalan and Prednisone (MP) and were analyzed for serum p53M level from the time of diagnosis to response to therapy or death. A subgroup of nine patients was regularly monitored for changes occurring in p53M levels during MP therapy. Serum levels of p53M were elevated in MM patients compared with MGUS and healthy controls (p = 0.002). Significantly higher p53M levels were shown by MM patients refractory to chemotherapy than by responding patients (0.38 ng/ml vs 0.22 ng/ml, p = 0.05). The measurement of serum p53M in the nine patients during the course of chemotherapy correlated with disease progression or response to therapy. If confirmed on a larger series of patients, these results suggest a potential role of serum p53 mutant levels in laboratory management of PCD patients.