Methods for detecting parasitemia in chronic Trypanosoma cruzi infection are either insensitive or non quantitative. The polymerase chain reaction (PCR), used to detect parasite kinetoplast (k) minicircle DNA, has been shown to be virtually 100% sensitive and specific in chronically infected persons. This technique has now been modified to be quantitative by using a competitor DNA. The competitive PCR yields equal amounts of kDNA and competitor PCR products when they are mixed in equimolar ratios. Thus, the amount of parasites can be estimated from the quantity of competitor DNA at the equivalencypoint. Blood from 5 chronically infected mice gave results consistent with 3-260 parasites/ml., and blood from 1 chronically infected person yielded 4 parasitesjmL. These are the first quantitative estimates of parasitemia in chronic T. cruzi infection. This technique could be useful for studying the natural history of T. cruzi infectionand the response to therapy. © 1994 by The University of Chicago.