In an investigation to find the prevalence of the fragile X (Martin Bell) syndrome in a mental handicap hospital, chromosomal investigations were carried out in 196 males selected out of a total of 512, and also in 20 female patients who were related to some of the selected males. Fragile X cells were found in 41 of the males and two of the females; in 21 of the males it was associated with macro-orchidism. The overall prevalence in the hospital for males (8.0%) ranks this syndrome next in importance to Down's syndrome as a known cause for mental handicap.