Background: Fragile X syndrome is a common inheritable cause of intellectual disability (ID) and is characterised by a large number of CGG repeats at the gene "FMR1" located on the X-chromosome. It has been reported that this genetic mechanism may protect against malignant transformations. Methods: We extracted from the Finnish registry on persons with ID a cohort of 302 persons with a fragile X diagnosis during 1982-1986. Follow-up for cancer incidence was performed in the Finnish Cancer Registry until the end of the year 2005. Results: There were 11 reported cancers during the mean follow-up of 21.4 years per person. The expected number of cancers based on the average Finnish population was 13.8 and no statistically significant protective effect was detected [standardised incidence ratios (SIR) 0.80, confidence interval (CI) 95% 0.40-1.4]. An increased risk for lip cancer was found (SIR 23, CI 95% 2.8-85). Conclusions: Confirmation of hypotheses about the mechanisms linking FXS and cancer needs further research.