The authors report the case of a young 35 year-old male patient, investigated due to thrombocytosis for three years. First the diagnosis of chronic myeloproliferative disease was made. The diagnosis of familial adenomatous polyposis was only evident in advanced stage of the disease. Upper abdominal US, abdominal CT, double-contrast barium enema examination and colonoscopy proved advanced synchronous colorectal cancers (sigmoid and descending colon) with liver metastases along with polyposis throughout the whole large bowel. Days after the diagnosis was made the patients condition deteriorated rapidly and he died with septic symptoms suggesting bowel perforation and pneumonia. Beside the case report the authors try to give a short overview of the current literature of relatively rare but potentially fatal hereditary colon cancer syndromes to awake the attention of the clinicians to investigate more cautiously the background of unexplained clinical-laboratory signs in young adults.