['Microsatellite instability is a property of most tumors occurring in the context of hereditary nonpolyposis colon cancer. Instability also occurs in 10%-15% of apparently sporadic colorectal cancers, and it has been hypothesized that this instability may indicate a genetic predisposition to colonic cancer. This study evaluated whether there is a clinically useful association between colon cancer instability and a family history of cancer.', 'Colon cancer cases (n = 188) from a population-based study were evaluated for microsatellite instability with 10 polymerase chain reaction primer sets. Instability results were compared with family history and other clinical and biological characteristics.', 'Microsatellite instability was found in 16.5% of tumors. It was predominantly a feature of right-sided tumors (P = 0.003) and was associated with the youngest and oldest ages at diagnosis (P = 0.01). Instability was not associated with family history of cancer, sex of the individual, or the glutathione-S-transferase mu 1 null genotype.'] Is microsatellite instability in human colonic cancer a useful clinical indicator of familial colorectal cancer?