Primary microcephaly is thought to result from genetic defects of the developmental program that generates large brain hemispheres in humans. Autosomal recessive inheritance is likely in most familial cases, and four loci were recently mapped by homozygosity. We report homozygosity mapping of a new locus, MCPH5, with a maximum multipoint LOD score of 3.51 at marker D1S1723, in a family of Turkish origin. The minimal critical region spans 11.4 cM between markers D1S384 and D1S2655, at 1q25-q32, and encompasses the cytogenetic breakpoints of chromosomal aberrations previously reported in unrelated patients with microcephaly.