Maternal uniparental disomy was observed in a 4-year-old boy with severe pre- and postnatal growth retardation (body height: 85 cm = 12 cm < third percentile, head circumference: 48 cm = 10 cm < third percentile), a few minor facial findings, and with apparent hyperactivity. His intelligence is within the normal range for his age. Karyotype analysis revealed two cell lines, one apparently normal with 46,XY, the other with a tiny marker (47,XY, + mar). Microdissection and reverse chromosome painting using the marker DNA library as a probe, as well as PCR analysis revealed that the marker is from chromosome 20 and contains only the centromere and pericentromeric segments, but none of the pericentromeric loci for microsatellites. Microsatellite analysis of 25 chromosome 20 loci disclosed maternal uniparental disomy for all 16 informative markers. Maternal heterodisomy was evident for seven loci of the short arm segment 20p11.2-pter. Maternal isodisomy was found at five loci, three of them map to the proximal 20p11.2 segment and two to 20q. To our knowledge, this is the first case of maternal disomy 20 in humans.