Inheritance of brachydactyly type D (BD-D) was studied in two unreported Caucasian family pedigrees and in 36 previously reported family pedigrees. The inheritance pattern was characteristic of a single autosomal dominant gene with incomplete penetrance. Using the proportion of individuals exhibiting the trait as the index, penetrance was sex influenced, being complete in females and approximately 62 percent in males. Expression of BD-D in both females and males was bilateral (both thumbs) for approximately three-fourths and unilateral (either right or left thumb) for approximately one-fourth of the individuals.