Neonatal leukemoid reaction. An isolated manifestation of mosaic trisomy 21.

A phenotypically normal neonate with an unexplained leukemoid reaction had a 46,XX/47,XX, +21 karyotype limited to the hemopoietic system. Although a self-limited myeloproliferative syndrome occurs in Down's syndrome, this case confirms that an identical disorder may develop in patients with mosaic trisomy 21 and no phenotypic stigmas of Down's syndrome. Chromosome analysis is a necessary adjunct to the examination of any neonate with an unexplained leukemoid reaction. The presence of trisomy 21 should lead to conservative management, unless there is life-threatening progression of the disease.