The clonal origin of multiple tumors in the same individual has long been debated. The main aim of this study is to find out whether multiple tumors in same individuals originated from a single clone. In our previous work (Pathol. Res. Pract. 199 (2003) 313-321), the deletion at chromosome1p36 was found to occur early because of common allelic loss in the bilateral tumors. In order to further investigate the findings about the clonality of tumors, eight tumors from four patients (two synchronous bilateral breast carcinoma [biBC], one case with breast carcinoma in one breast and multiple calcified fibroadenoma nodules in another breast, and one case with multifocal fibroadenosis in one breast) were subjected to polymerase chain reaction (PCR) to detect (a) loss of heterozygosity (LOH) and microsatellite size alterations (MA) using microsatellite markers distributed over five chromosomal arms 11p/q, 13q and 17p/q, and (b) Cyclin D1 amplification. Some markers were intragenic for BRCA1, BRCA2, BRCAX, ATM, TP53, and RB1. Although a few cases were studied, our findings suggest that in at least a proportion of patients multiple tumors may arise from a single clone.