We report a new case of partial trisomy 18q due to a balanced reciprocal translocation 4;18 in the mother. The female infant had a partial trisomy of the long arm of chromosome 18 associated with a partial monosomy of distal 4q. The infant showed many of the main clinical features of trisomy 18, such as dysmorphic face, congenital heart defect, crossing of the second and fifth fingers over the third and fourth with flexion contractures, and abnormal genitalia. We believe that the trisomy 18 phenotype requires a large region of 18q and that the greater the trisomic fragment, the more severe the expression. We stress the importance of genetic counseling to carriers of balanced translocations. The risk for each case should be evaluated and information should be given on the possibility of prenatal diagnosis.