Microsatellite alterations observed in tumor specimens may reflect genomic instability due to defective mismatch repair genes. To investigate whether this occurs in human nonsmall cell lung carcinomas we have analyzed microsatellite instability at 5 marker loci and loss of heterozygosity (LOH) at the hMLH1 locus on chromosome 3p21 using the polymerase chain reaction. Of a total of 49 nonsmall cell lung carcinomas examined, 43% (13 of 30 informative cases) showed LOH at 3p21 and 29% (14 of 49) exhibited microsatellite instability at one or multiple loci. LOH of the mismatch repair gene hMLH1 at 3p21 occurred in 82% (9 of 11 informative cases) of the tumors with microsatellite instability. This suggests that defects in the mismatch repair gene hMLH1 at 3p21 may be involved in microsatellite instability and tumorigenesis of a subset of nonsmall cell lung carcinomas. However, in those nonsmall cell lung carcinomas without microsatellite instability LOH at 3p21 probably involved another tumor suppressor gene(s) in this chromosomal region.