Trisomy 13 was diagnosed with genetic amniocentesis in a fetus of a 50 years old patient. Fetopathologic examination has shown cyclopy, proboscis and semilobar holoprosencephaly of the fetus, which is consistent with Patau syndrome. DNA was extracted from frozen liver tissue. Result of comparative genomic hybridization (CGH) was consistent with trisomy 13. They processed the DNA according Kallioniemi's method with modifications. CGH was developed for cancer genetics in mid 90s and now it is widely used in prenatal diagnosis too. CGH allows global analysis to detect unbalanced chromosome gains and losses in the whole genome in a single experiment without the need for cell culture. Significant results can be expected in those cases where conventional cytogenetics is not able to provide an answer either because postmortem tissue is not appropriate for cytogenetics or because the chromosomal change is sub-microscopical. CGH is a fluorescent in situ hybridization on a healthy target metaphase, with equal amount of competitive hybridization of green labelled digested test DNA and red labelled digested control DNA. Red to green ratio is assessed with the help of an image analyser. Green dominance represents chromosome gain, while red shift chromosome loss. In the paper they present the fetopathologic report of a trisomy 13 fetus and illustrate the method being the first Hungarian obstetric case diagnosed by CGH.