Maternal uniparental disomy (UPD) for chromosome 14 [upd(14)mat] may cause a characteristic phenotype with growth and developmental deficiency and precocious puberty. We report the case of a Japanese infant with an isochromosome 14 [i(14q)] and intrauterine growth retardation (IUGR). The infant is one of triplets comprising a boy (the patient) and two karyotypically normal girls. We analyzed parent-child transmission modes of alleles on the i(14q) at 17 CA-repeat marker loci along the entire length of chromosome 14. Genotypes at 4 proximal and 5 distal loci on the i(14q) were consistent with maternal isodisomy, whereas those at an intervening region indicated maternal heterodisomy. Thus, the derivative chromosome 14 had arisen through a translocation between maternal homologous chromosomes 14 [t(14;14)(p10;q10)] after at least two crossing-over events at the first meiosis. This result also suggests that there must be maternally imprinted gene(s) on 14q, and that loss of the functionally active, paternally derived allele in the same locus may lead to IUGR. Alternatively, IUGR may be an autosomal recessive trait. In the latter case, the mother would be a heterozygote and the putative disease locus would be either at the most proximal or most distal region of 14q.