Levels of circulating DNA increase under treatment by an artificial kidney. Using a new assay, levels of plasma DNA are studied in 45 patients during 99 sessions of hemodialysis or hemofiltration. Before the session, plasma DNA levels are increased in 41/99 samples and, among them, in 18/24 samples collected from hepatitis B surface antigen carriers. During the first 3 h of the session, plasma DNA levels increase whatever the method of treatment. At the 30th and 60th minute of hemodialysis, a positive gradient of plasma DNA exists between the output and the input of the artificial kidney. It is concluded that: (1) the increase in plasma DNA is related to the overall procedure of artificial kidney therapy; (2) death of leukocytes in the artificial kidney is responsible for the release and the increase in circulation of extracellular DNA.