Instability of dinucleotide tandem repeat sequences has been reported to play a major role in the carcinogenic pathway of familial colon cancer, as well as a potential role in the carcinogenesis of other sporadic neoplasms. To determine the frequency of short tandem repeat instability in adenocarcinoma of the prostate, we studied 40 tumors that were stratified according to tumor grade. The tissue samples were screened with di-, tri- and tetranucleotide markers spanning a wide range of chromosomal loci, including an androgen receptor gene trinucleotide repeat. Microsatellite instability was observed overall in only one of the 40 (2.5%) prostate adenocarcinomas studied. This replication error-positive tumor demonstrated repeat length alterations at two loci. Five other tumors showed an alteration in microsatellite size at a single locus. These tumors were not considered to have the microsatellite instability phenotype. All changes were identified either within tetranucleotide sequences or within the androgen receptor gene repeat (4 or 20 total markers analyzed). Both repeat length expansions and contractions were identified. The replication error-positive case also included separate metastatic nodal tissue. Additional microsatellite analysis of the metastatic tumor tissue revealed allelic patterns identical with the normal tissue control. Our data indicate that microsatellite instability is rare in prostate adenocarcinoma. Therefore, observation of this low replication error frequency suggests that most prostate carcinomas develop in the absence of widespread accumulation of somatic mutations in short tandem repeat sequences. Additionally, these genetic alterations appear to occur more often in tetranucleotide repeat sequences as well as in an androgen receptor gene trinucleotide repeat.