This study examined the prognostic value of circulating peripheral blood plasma cells (PBPCs) in patients with primary systemic amyloidosis (AL). A sensitive slide-based immunofluorescence technique was used to assess 147 patients for circulating PBPCs. Circulating monoclonal plasma cells were quantified as a percentage of circulating cytoplasmic immunoglobulin-positive cells (PBPC%). The absolute circulating plasma cell count was also determined. When analyzed retrospectively, 24 (16%) of 147 patients were found to have detectable circulating PBPCs. Overall survival for patients with high PBPC%'s (> 1%) was poorer (median survival, 10 vs 29 months; P =.002). Similarly, overall survival for patients with high PBPC counts (> 0.5 x 10(6)/L) was significantly poorer (median, 13 vs 31 months; P =.003). Increased percentages of bone marrow plasma cells (BMPC%; P =.0004), increased levels of serum beta(2)-microglobulin (P =.04), and dominant cardiac amyloid involvement (P =.03) also predicted poorer survival. The combined consideration of circulating PBPCs and BMPC% identified low-, intermediate-, and high-risk groups with median survivals of 37.5, 15.5, and 10 months, respectively (P =.0003). Multivariate analysis revealed circulating PBPCs and BMPC% to be independent prognostic factors for survival. Patients with PBPC%'s of 2% or higher were significantly more likely to have a coexisting clinical diagnosis of multiple myeloma (50% vs 12%, P =.008). The prognostic value of circulating PBPCs may help select treatment for patients with AL.