We present a new case of 11p15 duplication (trisomy 11p15) in a boy (46,XY,-21,+der(21), t(11;21)(p15.2;q22.3)] suffering from Beckwith-Wiedemann syndrome (BWS), whose phenotypically normal father carries a balanced translocation between chromosomes 11 and 21[46,XY, t(11;21)(p15.2;q22.3)]. The paternal grandmother has the same balanced translocation and is also clinically normal. BWS was suspected when the boy was 6 months old because of gigantism, macroglossia, visceromegaly, ear lobe creases and abdominal distention. Apart from the characteristic BWS phenotype, the boy has other features which are almost exclusively observed in 11p trisomy (high forehead with frontal upsweep of hair, wide central nose bridge, slightly beaked nose, chubby cheeks and severe mental retardation). So far, at least eight cases of 11p15 duplication have been described as patients with BWS. In six of these, the duplication was due to inheritance of a translocated or rearranged paternal chromosome. This was also the case in our patient. In the two other previously published cases, the 11p15 duplications were de novo, but in one of these, DNA analysis has subsequently shown that the duplication was of paternal origin. We discuss our observations in relation to the above-mentioned previous cases of 11p15 duplication and the possible role of genomic imprinting in the etiology of BWS.