This paper reviews non-specific X-linked mental retardation (X-linked MR), a heterogeneous group of X-linked recessive disorders which are controlled by one or more genes located on the X chromosome. Presently, there are at least three major subdivisions of X-linked MR. The IQ of affected males is variable. Carrier females usually have normal intelligence but some do show partial expression resulting in retardation. Genetics units can diagnose affected males utilizing pedigree analysis, clinical evaluation and cytogenetic testing (fragile X-linked MR type only). Projections indicate X-linked MR may be as common in males as Down syndrome and could account for 25% of the retarded male population.