Direct two-generation transmission of an unbalanced 18p+ chromosome was discovered after amniocentesis. Neither the mother nor the child exhibited apparent physical malformations or mental impairment. Banding analysis suggested a complete 18p duplication. Molecular studies verified the 18p origin of the duplicated material. Only 14 previous cases of duplication 18p have been reported and these exhibited either a normal phenotype or mild and inconsistent abnormalities. The present cases, as well as the review of literature, indicate that duplication 18p is associated with few or inapparent phenotypic abnormalities.