We established a database of hereditary multiple exostoses for the state of Washington, on the basis of a retrospective review of the medical records and a clinical evaluation of family members, to determine the prevalence, clinical range of expression, and rate of malignant degeneration. The database comprised forty-six kindreds with 113 affected members; all kindreds had at least one member living in the state of Washington. The over-all prevalence was at least one in 50,000. Approximately 10 per cent of the subjects had no family history of multiple exostoses. With the use of twenty-three pedigrees that demonstrated an adequate multigenerational history for determination of penetrance of the gene, we identified one unaffected individual among twenty-six obligate heterozygotes, a rate of penetrance of 96 per cent. There was no evidence for a substantial reduction of penetrance in female subjects. The median age at the time of the diagnosis in the 113 affected individuals was three years (range, birth to twelve years). in a cohort of eighty-four subjects for whom we had complete information, the clinical range of expression was wide: thirty-three (39 per cent) had an obvious deformity of the forearm, eight (10 per cent) had an inequality in the lengths of the limbs, seven (8 per cent) had an angular deformity of the knee, and two (2 per cent) had a deformity of the ankle. The average number of operations for the patients for whom the operative history was known was two. Of the forty-two subjects for whom function could be assessed, eleven (26 per cent) had a rating of poor; nine (21 per cent), fair; six (14 per cent), good; and sixteen (38 per cent), excellent. At the time of writing, chondrosarcoma had been diagnosed in only one (0.9 per cent) of the 113 affected subjects. However, seventy-six of these individuals were less than forty years old, so this rate may increase over time.
