Partial trisomy for the short arm and the proximal region of the long arm of chromosome 18 was observed in a male child of 4 years of age. The trisomy appeared de novo as a free, practically metacentric chromosome. A review of other cases of partial trisomy 18 suggests that there is a critical segment of the chromosome responsible for typical phenotype of the trisomy 18. In this report we add the description of a new case to the work already carried out in the delimitation of this critical segment.