The occurrence of cancer was studied in a cohort of 223 persons with fragile X syndrome, based on information from the Danish Cytogenetic Registry and the Danish Cancer Registry. Four cases of cancer were found, (carcinoma of pancreas, urinary bladder, uterus, and rectum). None of these cancer types have been reported in patients with fragile X syndrome previously. Compared with cancer rates in the general population, the standardized incidence ratio (SIR) was decreased to 0.28 (95% confidence interval: 0.1-0.8). The possibility that the expanded trinucleotide repeats in fragile X syndrome protect against cancer is discussed. © 2001 Wiley-Liss, Inc.