A new born male and a three-year-old female with various dysmorphic features were both found to have to have a supernumerary chromosome. Clinical and cytogenetic findings confirmed the existence of a pure de novo 9p tetrasomy in the first case and a pure de novo 9p trisomy in the second case. Gene dosage effects were demonstrated for galactose-1-phosphate uridyltransferase GALT (EC 2.7.7.12) using an improved method for the assay of this enzyme in red blood cells. These findings are in agreement with previous results on similar cases and therefore confirm the assignment of the locus for GALT to 9p.