In multiple myeloma (MM) an increase in circulating lymphocytes expressing plasma cell-associated antigens (PCAA) has been described. Its prognostic significance was evaluated in this study. The immunologic phenotype of peripheral blood lymphocytes was analyzed with a panel of monoclonal antibodies specific for B, T, natural killer lymphocytes, and PCAA (CD38, PCA1) in 52 MM patients at diagnosis, remission, and during relapse, 18 monoclonal gammopathy of undetermined significance (MGUS), and 25 normal controls. No significant phenotypic alteration was observed in MGUS. In MM, the number of B lymphocytes was in the normal range at diagnosis and during the subsequent phases. A CD4/CD8 ratio decrease, during relapse, was due to both a CD4+ reduction and to an expansion of a subset of CD8+ activated suppressor lymphocytes. CD38+ and PCA1+ lymphocytes at diagnosis were significantly higher than in MGUS, and a further increase was observed during relapse, suggesting a correlation between PCAA expression and disease activity. The prognostic significance of increased PCAA was confirmed by a survival analysis of 32 patients evaluated at diagnosis using a CD38 cutoff of 0.45 x 10(9)/L positive lymphocytes. Median survival for patients with high values was only 14 months, whereas it was not reached at 32 months by those with low values (P less than .0007).