In 1985, Mardini and Nyhan described three patients from consanguineous families with unilateral complete/partial lung agenesis, congenital cardiac defects, and ipsilateral thumb anomalies. Although there have been many reports of lung agenesis with other malformations, especially hemifacial microsomia and radial ray anomalies, very few demonstrate this triad of defects. We describe three patients with the Mardini-Nyhan association which may represent a distinct entity, although this remains uncertain at present. A fourth patient is also described, the sister of one of the other patients, with complex congenital cardiac disease and bilateral lung lobation abnormalities. This is the first reported incidence of a possible recurrence within a family and suggests, together with the consanguinity observed by Mardini and Nyhan, that recessive inheritance should be considered in genetic counseling for this disorder.