Attempts to study the genetics of human thumb polydactyly have been hampered by lack of awareness of the extremely varied expression of upper limb preaxial anomalies. It has been appreciated that thumb polydactyly could range from a broadened distal phalanx to complete duplication of the entire thumb. Most cases are sporadic and unilateral, but rare familial cases with wide variability and occasional nonpenetrance have been described. Four unrelated families are described who have thumb polydactyly as part of the range of expression for a dominant gene that is frequently associated with absence of thenar intrinsic muscles and flexor pollicis longus with inability to flex the thumb across the palm (the Fromont anomaly). These families and previous literature reports suggest that expression of the gene can range from thumb hypoplasia (most commonly the Fromont anomaly) to triphalangeal thumb or thumb polydactyly. As a consequence of this experience, we urge that parents, siblings, and other close relatives of patients born with thumb polydactyly be carefully examined for mild degrees of thumb hypoplasia or any other thumb anomaly, and that these findings be considered when providing recurrence risk counseling.