We have analyzed losses of heterozygosity (LOH) at markers from chromosomes 1, 2, 4 and 5 in a panel of 53 consecutive breast carcinomas. Together with a parallel analysis of LOH at chromosome 3, this allowed the identification of twenty-one regions of LOH. In contrast, in a comparative analysis of chromosome X, no region of loss could be defined. Cumulative regional allelotyping of 38 tumors with 71 markers from the five autosomes (37% of the genome) enabled the study of the respective distribution of regions of frequent loss, to identify associations of regions of LOH, and to distinguish tumors frequently affected by LOH such as lobular carcinomas.