When the darker eye has the smaller pupil.

Waardenburg and congenital Horner syndromes are both recognized causes of congenital hypochromic iris heterochromia. Each has been linked to disruptions in the pathway of tyrosinase induction, thus leading to a deficiency in melanin production of the iris. These syndromes must be considered in the differential diagnosis of a patient presenting with heterochromia iridis. We present the case of a 20-month old boy afflicted with both congenital Horner syndrome and Waardenburg syndrome, type II. In contrast to the more common presentation of congenital Horner syndrome, the affected iris in this case was the darker of the two because of the effects of the concomitant Waardenburg syndrome on the contralateral iris pigmentation. We are unaware of any other cases presenting with both Horner and Waardenburg syndromes and believe that this case serves as an excellent opportunity to briefly review the pathophysiology involved with these disorders.