We sought to determine whether circulating tumor cells in the blood stem cell harvest from patients with multiple myeloma are associated with a shortened disease-free survival. Prospective analysis was performed in 33 patients of blood obtained at leukapheresis for future transplantation. An immunofluorescence microscopy procedure identified the tumor cells by their morphology and monotypic light chain staining. Eighteen patients had increased (> or = 0.2 x 10(6)/l) monoclonal plasma cells circulating in the blood at stem cell harvest. Fifteen of the 18 have relapsed, with a median relapse-free survival of 6.2 months. Of 15 patients with < 0.2 x 10(6) cells/l, seven have relapsed, with a median relapse-free survival of 22.5 months (P = 0.008). Patients with circulating plasma cells showed a trend toward shorter overall survival (P = 0.078). In a multivariate analysis using the bone marrow plasma cell labeling index and beta 2-microglobulin, the absolute number of plasma cells in the stem cell harvest achieved borderline significance for predicting relapse-free survival (P = 0.057). In conclusion, increased monoclonal plasma cells in the blood stem cell harvest are associated with a shortened relapse-free survival. This does not necessarily indicate that the circulating plasma cells were responsible for relapse. These results, however, have implications with regard to the timing of obtaining blood stem cells for patients who are candidates for ablative chemotherapy.