Primary microcephaly is a clinical diagnosis made when an individual has a head circumference of greater than 3 standard deviations below the age and sex matched population mean, mental retardation but without other associated malformations and no apparent aetiology. The majority of cases of primary microcephaly exhibit an autosomal recessive mode of inheritance. We now demonstrate the genetic heterogeneity of this condition with the identification of a second primary microcephaly locus (MCPH2) on chromosome 19q13.1-13.2 in two multi-affected consanguineous families. The minimum critical region containing the MCPH2 locus is defined by the polymorphic markers D19S416 and D19S420 spanning a region of approximately 7.6 cM.