The paper is focused on guidelines of practice in inherited medullary thyroid cancer, diagnosed on the basis of DNA analysis. Identification of RET mutation implies further steps of diagnostic procedure, some of them - USG, FNAB and calcitonin level tests - are common for all types of mutation, other are related to ascertained type of mutation. In asymptomatic RET mutation carriers, prophylactic thyroidectomy is indicated. In MEN2B inherited cancer reveals its symptoms quickly and shows dynamic progress. In MEN2A/FMTC the clinical picture is diversified - in some patients the course of disease is mild, however in some other cases the progression of disease and even death occur regardless of the proper treatment. Unfortunately, there are no molecular prognostic markers in medullary thyroid carcinoma. Recent papers and also our own unpublished results show that gene expression profile, is similar in MEN2A and sporadic cancer. This group differs from MEN2B by its expression profile. In conclusion it is to be emphasized that although inherited medullary thyroid carcinoma is a rare disease, the diagnostic algorithm is well established and maximizes the chance for early diagnosis. Moreover, it needs to be stressed that DNA analysis results inform us not only about the necessity of further therapy, but also suggest different ways of proceeding in particular type of mutation.