Genetic, clinic and biologic features from three families with Xq fra syndrome are described. First, the authors give their experience for the best ways of culture in order to see the fragile site. Then, they insist upon the main points of each family: in the first, two women have a mental retardation; in the second, a prenatal diagnosis could be made; in the third, treatment with folic acid did not change the IQ but improves the behaviour of two brothers.