Heterozygous loss of relatively large chromosomal regions is a hallmark of the inactivation of tumour suppressor genes. Searching for deletions in cancer genomes therefore provides an attractive option to identify new tumour suppressor genes. Here, we have performed a genome-wide survey for regions exhibiting allelic loss in 24 commercially available breast cancer cell lines and four breast cancer xenografts, using microsatellite analysis. The assembled allelotype revealed an average fractional allelic loss of 0.34. A total of 19 arms had low allelic loss frequencies (<25%) and 17 arms had moderate allelic loss frequencies (25-50%). Five chromosomal arms were deleted in more than half of the breast cancer samples (8p, 10q, 13q, 17p, and 17q). Three of these frequently lost chromosomal arms had not been identified as such by comparative genome hybridisation, illustrating the higher sensitivity of microsatellite analysis for the detection of allelic losses. As we present allelic loss data of individual samples, our allelotype should not only aid the identification of new breast cancer genes but also provides a baseline for myriad studies involving these breast cancer cell lines.