We present an unusual case of trisomy 18 due to a pseudodicentric chromosome 18 of paternal origin. The karyotype was: 46,XY, -18, +psu dic(18)(qter-->cen-->p11.31::p11.31-->psucen-->qter). The origin of the abnormal chromosome was verified by FISH with a painting probe from chromosome 18. Absence of short-arm telomeres was shown by multicolor FISH, and the results of DNA analysis showed monosomy for loci D18S59 and D18S170 as well as paternal inheritance of the aberrant chromosome. The child's phenotype was characteristic of trisomy 18.