All human chromosomes were screened in 52 human breast carcinomas for the occurrence of allele losses, in order to identify genomic alterations involved in initiation and progression of the disease. Loss of chromosome 22 alleles was detected in 6 out of 8 lobular carcinomas, while chromosome 17 losses were most frequent in ductal carcinomas. Furthermore, patients who developed advanced disease after many years of mild clinical course showed significantly higher frequencies of allele losses in their primary tumors, compared to patients with a persistently mild disease course. Finally, in one case, molecular examination suggested a translocation t(10;17) with coamplification of the ERBB2 oncogene and chromosome 10 sequences present in the two tumors from this patient, consistent with one of the tumors being a metastasis originating from a subclone of cells in the other tumor.