Frequent allelic losses of chromosome 3p in lung cancer have been reported in a number of studies, and we previously demonstrated that 3p21.3 is one of the common regions of deletion in lung cancers and renal cell carcinomas. To further define a region containing the putative tumor suppressor gene, we performed Southern-blot analysis of 26 small cell lung cancer (SCLC) cell lines and ten non-small cell lung cancer (NSCLC) cell lines with 40 cosmid markers located at 3p21.3-p22. One marker detected homozygous deletions of four SCLC cell lines and one NSCLC cell line. None of the other markers revealed homozygous deletions or chromosomal rearrangements in these cell lines. The region of homozygous deletion described here is estimated to consist of less than 1 megabase of DNA, and it is very likely to contain at least one of the tumor suppressor genes associated with carcinogenesis of lung cancer and, possibly, renal cell carcinoma.