Twelve expert observers from nine different countries convened in a workshop to evaluate the validity of the Breast Cancer Conservative Treatment. Cosmetic results (BCCT.core) software, an objective method for the aesthetic evaluation of breast cancer conservative treatment. Experts were initially asked to subjectively classify the aesthetic results of 30 photographed cases submitted to breast cancer conservative treatment according to the four-point Harris scale. It was pre-established that if at least two-thirds [Cardoso MJ, Cardoso J, Santos AC, Barros H, Oliveira MC. Interobserver agreement and consensus over the esthetic evaluation of conservative treatment for breast cancer. Breast 2005] of participants provided the same classification this would be considered a consensual evaluation for that case. For cases where such agreement was not reached, consensus was obtained using a nominal group technique. Experts then individually performed objective evaluation of the same set of photographs using the BCCT.core software. This provides an automatic rating of aesthetic results, once scale and reference points in the photograph have been chosen. Agreement between observers, between each observer and the consensus, for computer evaluation obtained by the different participants and between software and consensus was calculated using multiple kappa (k) and weighted kappa (wk) statistics. In the subjective assessment, first-round consensus was achieved in 17 (57%) cases. Overall interobserver agreement was fair to moderate (k=0.40, wk=0.57). In the objective assessment there was a higher level of concordance between participants (k=0.86, wk=0.90). Agreement between software and consensus classification was fair (k=0.34, wk=0.53), but was higher in the 17 cases that reached first-round consensus (k=0.60, wk=0.73). Merging the two middle classes of the Harris scale, to form a three-point scale, led to an improvement of all non-weighted measures of agreement. These results show that the BCCT.core software provides consistent evaluation of cosmesis. It has the potential to become a gold standard method for assessment of breast cosmesis in clinical trials, as it can be used simultaneously by a panel of observers from different parts of the world to provide more reliable assessments than has been possible previously.