The diagnosis of hereditary cancer syndromes is basing mainly on pedigree analysis and if possible on molecular studies. We present 2 families which were diagnosed with hereditary cancer syndromes. Family I - breast cancer syndrome; 9 breast cancer cases were diagnosed in women between 25 and 45-year-of-age from 4 generations. The BRCA1 5382insC mutation was found in family members. Family II - hereditary nonpolyposis colon cancer; altogether 17 cancers in 12 persons from 6 generations were diagnosed. The c. 256 C>T mutation in MLH1 gene was found in family members. Thus, written family genetic counselings were prepared for both families, including detail information concerning cancer risk for carriers and non-carriers of critical mutation in given family. In conclusion it was stated that an effective prevention dedicated to patients at high, hereditary risk of cancer could be achieved by early diagnosis and thus by highly targeted surveillance and managements.