Data on nonspecific mental retardation in two or more sibs ascertained from the British Columbia Health Surveillance Registry are utilized to calculate the frequency of X-linked mental retardation in the population of British Columbia (B.C.). In the 1950-1969 birth cohort there are 107 sibships with only males affected and 35 sibships with only females affected giving an excess of 72 male-affected sibships. Family histories of the sibships with two more more mentally retarded males provide evidence that the mothers of the excess of male-affected sibships are carriers of X-linked mental retardation. Thus, a frequency of 1.83 per 1,000 males for X-linked mental retardation and carrier frequency of 2.44 per 1,000 females are calculated. This frequency can account for all the excess of males in the population who are affected with nonspecific mental retardation. We estimate that seven to 19 X-linked genes exist to cause nonspecific mental retardation, and that the frequency of the marker-X chromosome, associated with X-linked mental retardation in some families, may be 0.92 per 1,000 male births.