Regional increases in DNA methylation occur in normally unmethylated cytosine-rich areas in neoplastic cells. These changes could potentially alter chromatin structure to inactivate gene transcription or generate DNA instability. We now show that, in human lung and colon cancer DNA, hypermethylation of such a region consistently occurs on chromosome 17p in an area that is frequently reduced to homozygosity in both tumor types. Over the progression stages of colon neoplasia, this methylation change increases in extent and precedes the allelic losses on 17p that are characteristic of colon carcinomas. We also show on chromosome 3p that regional hypermethylation may nonrandomly accompany chromosome changes in human neoplasia. Increased methylation is consistent in small-cell lung carcinoma DNA at two 3p loci that are constantly reduced to homozygosity in this tumor, but it is not seen in colon cancer DNA, in which these loci are infrequently structurally altered.Alterations in DNA cytosine methylation are one of the most consistent, but poorly understood, molecular changes in human cancers (reviewed in refs. 1 and 2). These abnormalities appear early in progression of tumors and reflect an imbalance in the DNA methylation process that includes widespread hypomethylation (3), regional areas of hypermethylation (4), and an increased cellular capacity to methylate DNA (5, 6). The regional hypermethylation may be particularly important. Our laboratory (4) and others (7,8) have found this change in clusters of CpG dinucleotides near regulatory areas of genes. A normally unmethylated status of these "CpG islands" appears to be essential for active gene transcription (reviewed in ref. 9). Conversely, cytosine methylation within CpG islands can either stabilize or cause alterations in chromatin structure that render chromosome regions transcriptionally inactive (for review, ref. 10) or subject to DNA instability (11).We now explore whether chromosomal regions that are distinctively altered in human cancers are nonrandomly associated with hypermethylation and how the timing of this abnormality relates to changes in chromosome structure during tumor progression. A CpG-island-rich area on chromosome 17p, consistently reduced to homozygosity in both lung and colon cancers, is extensively hypermethylated in both tumor types. During progression stages for colon neoplasia, this abnormal methylation occurs early, increases in extent, and precedes allelic losses of the involved region. We also find that regions of chromosome 3p that are consistently reduced to homozygosity in lung, but not colon, cancers are hypermethylated only in lung cancer. We suggest that DNA hypermethylation nonrandomly marks chromosome regions altered during the development of specific tumors.