Allelic loss studies have been instrumental in identifying tumor suppressor gene loci in a variety of cancers. In this study we analyzed prostate cancer specimens from 52 patients for allelic loss using 8 polymorphic probes for the short arm of chromosome 8. Overall, 32 of 51 (63%) informative tumors showed loss of at least one locus on chromosome 8p. The most frequently deleted region is observed at chromosome 8p22-8p21.2. Loss of one allele is identified in 14 of 23 (61%) tumors at D8S163, in 15 of 32 (47%) tumors at lipoprotein lipase, and in 20 of 29 (69%) tumors at MSR, all on 8p22. Loss of one allele is identified in 16 of 27 (59%) tumors at D8S220 at 8p21.3-8p21.2. In addition to frequent loss of one allele at the MSR locus, one metastatic prostate cancer sample demonstrated homozygous deletion of MSR sequences. Loci telomeric and centromeric to this region are largely retained. A chromosome 8p deletion map is constructed and defines the smallest region of overlap to a 14-cM interval at 8p22 between D8S163 and lipoprotein lipase, flanking the MSR locus. Evidence of chromosome 8q multiplication at locus D8S39 was detected in 5 of 32 (16%) tumors, all of which demonstrated loss with at least one probe on chromosome 8p. This study extends the previous finding of frequent loss of chromosome 8p in prostate cancer by defining a common region of loss of heterozygosity at 8p22 and a homozygous deletion of the MSR locus contained within this region. This is the first homozygous deletion identified in the genome of a human prostate cancer and the highest rate of loss yet reported on chromosome 8p in cancer. These results strongly suggest the presence of a tumor suppressor gene in this region which is frequently inactivated in prostate cancer.