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To describe the diagnostic performance of array comparative genomic hybridization (aCGH) in the presence of mosaicism in the fetoplacental unit using direct chorionic villi.In an ongoing study on the diagnostic performance of aCGH in 80 high-risk pregnancies, we studied three cases of placental mosaicism by carrying out aCGH on DNA of direct chorionic villi and chorionic villi cultures.Case 1: A three- to fourfold dosage gain of the region 18p in aCGH on direct villi was due to two additional isochromosomes 18p confined to the cytotrophoblast. Case 2: aCGH on direct villi revealed a normal result, whereas trisomy 18 mosaicism was present in cultured cells. Case 3: aCGH identifies monosomy X and mosaic disomy of the region Xp11.21-Xq12, whereas this mosaic cell line is not present in the conventional chromosome preparation on the cytotrophoblast.Although interpretation of aCGH results may be straightforward in the majority of cases, placental mosaicism may cause misinterpretations of rapid aCGH results on direct chorionic villi due to discrepant chromosomal constitutions of cytotrophoblast and mesenchymal villus core. Further investigations including cultures, fluorescence in situ hybridization and possible amniocentesis will still be required for interpretation of results. Copyright © 2011 John Wiley & Sons, Ltd.