The development of a phenotypic checklist for identifying people with fragile X syndrome is described. The checklist was designed to identify people with developmental disabilities of unknown causes for molecular genetic testing for fragile X syndrome. The list consists of 28 items (7 on physical characteristics and 21 on behavioral features). Validation data were collected for 110 boys and men with fragile X syndrome and for 79 members of a control group, matched for CA, level of cognitive development, and social (mal)adaptation. On the basis of checklist results, those boys who are likely to be diagnosed as having fragile X syndrome can be identified. The screening list can be considered to be a consistent, reliable, and valid instrument.