This invention relates to detection of specific extracellular nucleic acid in human or animal blood plasma or serum associated with disease. Specifically, the invention relates to detection of nucleic acid derived from mutant oncogenes or other tumor-associated DNA, and to methods of detecting and monitoring extracellular mutant oncogenes or tumor-associated DNA found in blood plasma or serum. In particular, the invention relates to the detection, identification, or monitoring of the existence, progression or clinical status of neoplasia in humans or other animals that contain a mutation that is associated with the neoplasm through detection of the mutated nucleic acid of the neoplasm in plasma or serum fractions.