Fragile X syndrome (FXS) is the most common cause known of inherited developmental disability. New and widely available techniques in DNA analysis allow both affected individuals and carriers to be tested from a simple blood sample. It is now recognised that a wide range of clinical severity occurs in both sexes. Specific treatment and interventional strategies are now available and of great benefit to individuals, families and carers. Genetic counselling allows families to make informed decisions. General practitioners can play a central role in detection and management of this syndrome.