OBJECTIVES: Mosaic autosomal duplications are rare and often result in mental retardation and congenital anomalies. Phenotype is not predictable depending on the chromosomal imbalance involved and the percentage and tissues distribution of unbalanced cells. We report on a young woman carrying a mosaic duplication of chromosome 4p, evaluated because of three abortions due to IUGR and fetal malformation. METHODS: Mosaic dup(4p) was detected by standard and molecular cytogenetics. RESULTS: Unbalanced cells accounted for about 20 to 30% of nuclei in four examined tissues and did not cause any obvious phenotypic effect. CONCLUSION: It is likely that mosaic duplications are underascertained because they are not associated with obvious clinical effects in some individuals. Prenatal diagnosis is the method of choice to predict the karyotype in the offspring of subjects carrying mosaic chromosome imbalances.