Triphalangeal thumb is a developmental anomaly, sometimes dominantly transmitted, characterized by a long, finger-like thumb with three phalanges instead of two. The underlying genetic defect is unknown, but presumably involves genes that regulate the differentiation of the developing forelimb. In two large kindreds with triphalangeal thumb, evidence for linkage to the long arm of chromosome 7 was obtained with a maximum lod score of 12.61. Multipoint linkage and haplotype analysis placed the gene close to the telomere of the long arm. To our knowledge this is the first time that a human gene involved solely in the pathologic morphogenesis of the hand and feet has been localized.