Objective To investigate the loss of heterozygosity (LOH) of 3p D3S1300 and D3S1289 loci of plasma DNA of lung cancer (LC) patients and evaluate its value for a tumor marker of lung cancer.Methods Two microsatellite markers,D3S1300 and D3S1289,were analyzed by PCR and silver staining to investigate LOH of the plasma DNA and cancer tissue DNA from 69 cases of primary lung cancer,and the plasma DNA from 40 control subjects.Results The percentages of D3S1300 LOH detected in the tumor tissue and plasma DNA of LC patients were 40.6% and 29.0% respectively,while the percentages of D3S1289 LOH were 31.9% and 24.6% respectively.In contrast,only 2 cases of D3S1300 LOH and 3 cases of D3S1289 LOH were found in plasma DNA of control group (P<0.01,compared with lung cancer group).Conclusion 3p D3S1300 LOH and D3S1289 LOH in plasma DNA may be as a tumor marker for diagnosis of lung cancer.