Loss of heterozygosity (LOH) in loci of the 10q23 region that harbor the PTEN gene and mutations in the sequence of this gene have been found in several primary human tumors including breast carcinomas, suggesting that this gene could be implicated in their pathogenesis. We investigated allelic losses in microsatellites of the 10q23 region, and their correlations with nine pathologic parameters in 105 breast carcinomas. The LOH analysis was performed by amplifying DNA by PCR, using five markers of the 10q23 region (D10S1687, D10S541, D10S2491, D10S583 and D10S571). LOH in at least one marker of the PTEN region was found in 29.5% of tumors. The statistical comparison between carcinomas with and without LOH in terms of the pathologic parameters showed significant differences in age (p = 0.03), lymph node metastases (p = 0.02), and higher histological grade (p = 0.02); a trend toward significance was found for progesterone receptors (p = 0.05). LOH in an individual marker and statistically significant relationships to tumor characteristics were observed at locus D10S541 for lymph node metastases (p = 0.04), at D10S2491 (intragenic to the PTEN gene) for lymph node metastases (p = 0.02), and at D10S583 for progesterone receptors (p = 0.01) and for high grade (p = 0.03). These results suggest the PTEN gene, or other genes of the 10q23 region, could be functionally related to breast cancer, probably influencing the development of histological features associated with poor prognosis.