Authors summarize in this brief review results of European discussion, held on ETA-CRN Meeting in Lisbon, 2009, on the American Thyroid Association Medullary Thyroid Cancer (MTC) Guidelines published in the same year and focus on the timing of prophylactic thyroidectomy. ATA 2009 guidelines classified RET protooncogene mutation carriers into 4 levels: A, B, C, D. ATA for prophylactic thyroidectomy were generally independent of the serum calcitonin (Ct) concentration but based on a priori risk levels. This was well accepted as the important novelty was to delineate risk level specially for RET 634 mutation (level C). In the ATA Guidelines total prophylactic thyroidectomy below age 5 years was recommended in RET 634 mutation carriers regardless of Ct status. However, some European experts favored to base the decision not only on the results of DNA testing but also on the going Ct level. The European discussion reflected divergent opinions and indicated the need of publication of European experience instead of arbitrary opinions. It was stressed that patients carrying the same RET mutation present heterogenic progression to the clinically overt medullary thyroid cancer, even in the same family. Thus, in summary, the ATA MTC guidelines constituted a positive stimulus to publish further evidence for Ct-guided pre-emptive thyroidectomy for RET gene mutation carriers and the conclusion is drawn on the basis of experience expressed in Lisbon and published later evidence that the integrated algorithm based on age - Ct - type of RET mutation should be considered in the decision of pre-emptive thyroidectomy.