Seckel syndrome is a rare autosomal recessive condition belonging to the group of osteodysplastic primordial "dwarfism" and characterized by the association of 1) severe pre- and postnatal growth retardation, 2) microcephaly with mental retardation, and 3) specific dysmorphic features. Recently, two disease loci have been mapped to chromosomes 3q22.1-q24 and 18p11.31-q11.2, respectively, by homozygosity mapping in consanguineous families. Here, we report on the exclusion of these loci in five consanguineous and one multiplex nonconsanguineous Seckel syndrome families and in two consanguineous families presenting type II osteodysplastic primordial dwarfism. These results support the view that Seckel syndrome is a clinically and genetically heterogeneous condition.