The cosmetic result after breast surgery is an important marker in clinical studies. Most authors used subjective scales to judge breast cosmesis. However, inter-observer discrepancies are very high and the use of such subjective scales for prospective trials is highly disputed. In this study we present for the first time a new invented breast symmetry index (BSI). This BSI is calculated by subtracting the size and the shape between both breasts (frontal view and side view). The BSI is measured with a software system called breast analysing tool (BAT) from digital photographs. The photographs of 27 patients have been analysed with this software by different physicians to evolve inter-observer reproducibility. The Harris scale for subjective cosmetic analyses has been correlated with the BSI. In our study the inter-observer reproducibility was excellent (Pearson correlation r=0.9; p<0.05) and the BSI was able to significantly differentiate between good and bad cosmesis (BSI values from 0%d to 30%d is good, BSI>30%d is bad cosmesis). Thus the BSI may be used for clinical studies.