Conjoined twins discordant for phenotype of cleft lip are reported. One had right-sided cleft lip, while the co-twin had left-sided cleft lip. Both twins had a normal 46,XY karyotype. Genotyping at 50 polymorphic loci revealed that the twins were identical (monozygotic). Our analysis suggests that the laterality of cleft lip is more affected by the process of twinning than by genetic factors.