To better understand whether replication-error-type instability (RER 1 ) is a frequent genetic alteration event in surgicalpathologic stage-I non-small-cell lung cancer (NSCLC) and identify whether it constitutes an independent prognostic parameter, we examined 35 surgical-pathologic stage-I-NSCLC patients with complete follow-up in all cases for at least 49 months. The tumor samples and the paired histopathologically normal lung samples for each patient were analyzed for 8 microsatellite markers located at chromosomes 3p and 2p to investigate microsatellite alterations such as RER 1 and loss of heterozygosity (LOH). Single-strandconformation-polymorphism analysis for detection of p53 and k-ras gene mutations was also carried out. Genetic data were correlated with clinical outcome and histopathologically established prognostic factors. RER 1 at one or both chromosomes was identified in 24 of the 35 patients; 9 patients showed LOH. A statistically significant correlation was found between RER 1 and poor prognosis (p 5 0.001). Furthermore, RER 1 proved to be an independent factor that predicted decreased survival, ranking first, followed by visceral pleural invasion. A trend towards worse survival was strongest in the group of patients with tumor size greater than 3 cm (T2). Patients with other genetic abnormalities, such as K-ras mutations, p53 mutations or LOH, had prognoses similar to those of patients without such aberrations. The data suggest that RER 1 is common in NSCLC, that it may provide important prognostic information in stage-I NSCLC and serve as a useful marker for relapse-risk assessment in operable NSCLC patients.