Deletions in the short arm of chromosome 17 (17p) involving the tumor suppressor TP53 occur in up to 20% of diffuse large B-cell lymphomas (DLBCLs). Although inactivation of both alleles of a tumor suppressor gene is usually required for tumor development, the overlap between TP53 deletions and mutations is poorly understood in DLBCLs, suggesting the possible existence of additional tumor suppressor genes in 17p. Using a bacterial artificial chromosome (BAC) and Phage 1 artificial chromosome (PAC) contig, we here define a minimally deleted region in DLBCLs encompassing approximately 0.8 MB telomeric to the TP53 locus. This genomic region harbors the tumor suppressor Hypermethylated in Cancer 1 (HIC1). Methylation-specific PCR demonstrated hypermethylation of HIC1 exon 1a in a substantial subset of DLBCLs, which is accompanied by simultaneous HIC1 deletion of the second allele in 90% of cases. In contrast, HIC1 inactivation by hypermethylation was rarely encountered in DLBCLs without concomitant loss of the second allele. DLBCL patients with complete inactivation of both HIC1 and TP53 may be characterized by an even inferior clinical course than patients with inactivation of TP53 alone, suggesting a functional cooperation between these two proteins. These findings strongly imply HIC1 as a novel tumor suppressor in a subset of DLBCLs.