Cytogenetic and molecular investigation of a boy with precocious puberty and motor developmental delay revealed a 45,XY,t(14q14q) or i(14q) karyotype with no paternal chromosome 14 contribution. VNTR analysis of loci on four other chromosomes excluded non-paternity with greater than 99% confidence. Results of VNTR and CA repeat analyses of ten loci along the entire length of chromosome 14 were consistent with homozygosity at all loci, suggesting that the chromosomal rearrangement was a maternal isochromosome for 14q. As the proband's father had a balanced Robertsonian translocation, t(13q14q), we suggest that the origin of the maternal uniparental disomy (UPD) was fertilization by a nullisomy 14 sperm with formation of the isochromosome in the early embryo. Also, the proband has several clinical features in common with six previously reported liveborn cases of maternal UPD 14: hypotonia and motor developmental delay, mild dysmorphic facial features, low birth weight and growth abnormalities, and, more specifically, precocious puberty among the four cases old enough to assess. The emergence of a syndrome associated with maternal UPD 14 suggests the possibility of genomic imprinting of regions of chromosome 14, especially a gene involved in the onset of puberty.