BRCA1 is a major gatekeeper of genomic stability. Acting in multiple central processes like double-strand break repair, centrosome replication, and checkpoint control, BRCA1 participates in maintaining genomic integrity and protects the cell against genomic instability. Chromosomal instability (CIN) as part of genomic instability is an inherent characteristic of most solid tumors and is also involved in breast cancer development. In this study, we determined the extent of CIN in 32 breast cancer tumors of women with a <i>BRCA1 </i>germline mutation compared to 62 unselected breast cancers. We applied fluorescence in situ hybridization (FISH) with centromere-specific probes for the chromosomes 1, 7, 8, 10, 17, and X and locus-specific probes for 3q27 <i>(BCL6)</i>, 5p15.2 (D5S23), 5q31 <i>(EGR1)</i>, 10q23.3 <i>(PTEN)</i>, and 14q32 (<i>IGH</i><i>@</i>) on formalin-fixed paraffin-embedded tissue microarray sections. Our hypothesis of an increased level of CIN in <i>BRCA1</i>-associated breast cancer could not be confirmed by this approach. Surprisingly, we detected no significant difference in the extent of CIN in <i>BRCA1</i>-mutated versus sporadic tumors. The only exception was the CIN value for chromosome 1. Here, the extent of CIN was slightly higher in the group of sporadic tumors.