The authors report a case of non-polyposis colon cancer in a seventeen year old female without prior polyposis or family history. Since it was the first case in this family, HNPCC was suspected. The polymerase chain reaction (PCR) of the tumor revealed changes in four polymorphic regions. Analysis of two of them, indicated the loss of genetic material confirming instability suggestive of HNPCC. The patient underwent ileorectal anastomosis and adjuvant chemotherapy with a good outcome. The authors discuss the importance of family history, genetic and immunohistochemistry studies, and the instability of microsatellites in adolescents with colorectal cancer.