A systematic search was made for uniparental disomy in carriers of apparently balanced chromosome translocations who also had unexplained abnormalities of mental or physical development. Of 65 families studied, biparental origin of both translocated chromosomes was demonstrated in 64, and only 1 case of maternal uniparental disomy of chromosome 14 was detected in the carrier of a Robertsonian t(13q14q). We conclude that uniparental disomy is a rare occurrence in this population.