The trisomy 8 mosaic syndrome (Tr8MS), karyotype 46,XY/47,XY, +8, is a rare multisystem disorder that may be associated with corneal opacity. We report the case of a dysmorphic infant with multiple congenital abnormalities referred to our unit with a congenital corneal opacity. Subsequent chromosomal analysis of peripheral leucocytes demonstrated constitutional Tr8MS. At 4 years of age, lamellar keratoplasty was performed. Histological examination confirmed the lesion to be consistent with a corneal choristoma. Cytogenetic studies using in situ hybridisation techniques showed the presence of trisomic cells in cell culture derived from the tissue in higher proportion (92%) than in the blood (44%). Amplification of the c-myc oncogene on chromosome-8 could not be detected in cells cultured from the corneal lesion. Although not proof, these findings lend support to the concept of the corneal lesion representing a focus of viable trisomic cells rather than an inflammatory response to a nidus of effete cells.