To determine whether p16 is altered in human malignant mesothelioma (MM), molecular analysis of multiple 9p loci was performed on 40 cell lines and 23 primary tumors from 42 MM patients. We identified homozygous deletions of p16 in 34 (85%) cell lines and a point mutation in 1 line. Down-regulation of p16 was observed in 4 of the remaining cell lines, 1 of which displayed a DNA rearrangement of p16. Homozygous deletions of p16 were identified in 5 of 23 (22%) primary tumors; no mutations or rearrangements were found in these specimens. Four cell lines displayed a single homozygous deletion proximal to or distal to p16; 4 others had 2 nonoverlapping deletions, one involving p16 and the other involving a region proximal to this locus. These data indicate that alterations of p16 are a common occurrence in MM cell lines and, to a lesser extent, in primary tumors. Furthermore, deletions of 9p21-p22 outside of the p16 locus may reflect the involvement of other putative tumor suppressor genes that could also contribute to the pathogenesis of some MMs.