We report on a stillborn male infant with a mosaic ring 13 karyotype (45,XY,-13/46,XY,-13,+r(13)) with apparent aprosencephaly and clinical findings similar to those reported previously in the XK-aprosencephaly syndrome. Findings of patients with r(13) are often similar to those seen in individuals with del(13q). This case was unusual because of the presence of aprosencephaly, although brain malformations such as arhinencephaly and cerebellar hypoplasia are present in at least one-half of reported patients with 13q-. The overlap between these syndromes suggests a possible chromosomal model of the XK-aprosencephaly syndrome.