Flow cytometric studies were performed on peripheral blood obtained before, during, and subsequent to urologic surgery. The results suggest that 7 of 11 patients had a brief period during surgical manipulation when they had cells from the primary surgical site in the peripheral blood. Concentration of such cells was generally less than 10(2)/mL even during the intraoperative period. These findings suggest that perioperative tumor seeding is a real concern, and that flow cytometry can be used to study maneuvers to reduce its incidence.