We report on a 23-year-old man with craniofacial findings of the holoprosencephaly spectrum disorder (microcephaly, hypotelorism, depressed nasal bridge, single median maxillary central incisor), fusion of C2–C3 vertebrae, intellectual disability, and severe sleep apnea. Chromosome analysis of blood lymphocytes showed 75% ring (18) cells and 25% normal cells, karyotype mos 46,XY,r(18)(::p11→q21::)/46,XY. His mother was phenotypically normal except for a double ureter and bifid renal pelvis as in his son. She had a supernumerary ring (18) in 10% of blood lymphocytes, karyotype mos 47,XX,+r(18)(::p11→q21::)/46,XX. Familial ring (18) is a rare cytogenetic abnormality. This is the first report of a mother with a supernumerary ring (18) and a son with ring (18) mosaicism. Interestingly, the son showed a true mosaicism (mixoploidy) of ring (18) and normal cells. The mother's 46,XX cells could be easily explained by mitotic instability and ring loss during cell division. However, the coexistence of ring (18) and normal cells in the son is unusual. Possibly, during early postzygotic divisions of a 47,XY,+r(18) zygote, two (possibly subsequent) genetic events could have occurred, one when one normal chromosome 18 was lost (resulting in a cell line with ring 18), and one when the ring 18 was lost (resulting in a cell line without ring, “escape to normal”). Alternatively, the zygote of the son could have been 46,XY,r(18), and postzygotic loss of the ring 18 could have resulted in monosomy 18 cells followed by duplication of chromosome 18 in these cells (a rare mechanism for cell survival previously described as “compensatory” isodisomy). © 2011 Wiley-Liss, Inc.