Previous cytogenetic investigations have revealed frequent deletions and other unbalanced structural rearrangements of 3p in human malignant mesothelioma. We have performed a restriction fragment length polymorphism analysis by using the polymerase chain reaction and primer sets for seven DNA markers to examine loss of heterozygosity (LOH) from 3p in 25 malignant mesotheliomas. Among 24 cases informative at one or more 3p loci, 15 (62.5%) exhibited LOH with at least one marker. Deletion mapping in these tumors indicates that the common region of chromosomal loss resides within band 3p21, in the vicinity of the D3F15S2 locus. These results suggest that allelic loss from 3p21 is a frequent occurrence in malignant mesothelioma and that one or more putative tumor suppressor genes at this site contribute to the pathogenesis of this malignancy.