Apparently the first patient with de novo mosaicism 46,XX,t(13q13q)/46,XX,-13,+r(13) is described. The two cell lines were present at a frequency of 34% and 66%, respectively. The infant survived for about three months. The prominent dysmorphic features were: birth-weight and head circumference below the 3rd centile, encephalocele, multiple skin tags of low set dysplastic ears, coloboma of the left iris, short upward slanting palpebral fissures, and prominent nasal root. An imperforate anus, recto-vaginal fistula, enlarged adrenals, missing/hypoplastic kidneys, and limb anomalies were also present. It is postulated that the ring is a secondary anomaly arising from the 13q13q translocation.