DMBT1 (deleted in malignant brain tumors) is a candidate tumor suppressor gene that has been mapped to chromosome 10q25.3-q26.1, a region in which frequent loss of heterozygosity (LOH) has been observed in several human tumors. Since DMBT1 is highly expressed in the lung, we analyzed LOH at the DMBT1 locus and expression of this gene in lung cancer. Thirty-five (53%) of 66 primary lung cancers showed LOH, and diminished expression of DMBT1 was observed in 20 (91%) of 22 lung cancer cell lines: three (14%) of them showed loss of expression. We further determined the primary structure of DMBT1 and analyzed genetic alterations in this gene using 23 lung cancer cell lines. Two (9%) of them had homozygous deletion within the gene, and two cell lines had genetic aberrations: one was a rearrangement involving exons 5 and 6, and the other was a missense mutation at codon 52. These results suggest that inactivation of the DMBT1 gene plays an important role in human lung carcinogenesis.