Trisomy 18 is the second most frequent autosomal aneuploidy affecting about 1 in 8,000 new-borns. Similar to trisomy 13 more than 90% of the patients die within the first year. Main causes of death are failure of vital organ function, in most cases of brain, heart, kidney, and gut, sometimes combined with severe infections. The degree to which essential organs are affected at birth and the clinical course differ considerably. Unknown genetic factors and various environmental effects are most likely involved. A less severe course of Edwards syndrome can be caused by a partial trisomy due to a deletion of the extra chromosome 18 or somatic mosaicism with a trisomic and a normal cell-line in the patient. In this report conventional chromosome analysis, FISH, and QF-PCR have been performed on a 19-year-old female patient with trisomy 18 to investigate a large number of cells including non-mitotic cells from various different tissues. This study supports evidence for an apparently pure form of trisomy 18 in this "long-living" patient with Edwards syndrome.