In the course of studying a case of cystic fibrosis, Spence et al. (1988) discovered what appeared to be a case of uniparental disomy: the father did not contribute alleles to the propositus for markers near the CF locus or for centromeric markers on chromosome 7. High-resolution cytogenetic analysis was normal, and the result could not be explained by nonpaternity or a submicroscopic deletion. Uniparental disomy could be explained by various mechanisms such as monosomic conception with subsequent chromosome gain, trisomic conception followed by chromosome loss, postfertilization error, or gamete complementation. Patients with more than one genetic disorder might be suspected of having isodisomy, which should also be suspected in cases of an apparent new mutation leading to a recessive disorder when only 1 parent is heterozygous, and in cases of females affected with X-linked recessive disorders. Engel (1980) appears to have originated the concept of uniparental disomy and resulting isodisomy. Voss et al. (1988, 1989) also demonstrated uniparental disomy for chromosome 7 in a patient with cystic fibrosis.