We have identified 39 X-linked conditions in which mental retardation seems to be the primary characteristic, although pathogenesis is unknown. These conditions can be subdivided into syndromal and non-syndromal, depending on the existence of a recognizable pattern of minor anomalies and/or malformations, or lack thereof. Seventeen genes have been regionally mapped onto the X chromosome. However, in 14 instances the data were derived from a single family and most lod scores were less than 3.0.