Primary palmar hyperhidrosis is a condition marked by excessive perspiration and is reported to have an incidence of 1% in the Western population. It is a potentially disabling disorder that interferes with social, psychological, and professional activities. Over the past several years, several investigators have reported a positive family history in their patients treated for hyperhidrosis. To date, the cause is unknown; furthermore, epidemiologic data are scarce and inadequate.To characterize the genetic contribution to hyperhidrosis, we conducted a prospective study of 58 consecutive patients with palmar, plantar, or axillary hyperhidrosis treated with thoracoscopic sympathectomy from September 1993 to July 1999. Forty-nine of the 58 probands volunteered family history data for these analyses (84% response rate). A standardized questionnaire was administered during the postoperative visit or by phone interview, and a detailed family history was obtained. The same questionnaire was also administered to a set of 20 control patients. The familial aggregation of hyperhidrosis has been quantified by estimating the recurrence risks to the offspring, parents, siblings, aunts, uncles, and cousins of 49 probands and 20 controls. We estimated the penetrance by use of a genetic analysis program.Thirty-two of 49 (65%) reported a positive family history in our hyperhidrosis group, and 0% reported a positive family history in our control group. A recurrence risk of 0.28 in the offspring of probands compared with frequency of 0.01 in the general population provides strong evidence for vertical transmission of this disorder in pedigrees and is further supported by the 0.14 risk to the parents of the probands. The results indicate that the disease allele is present in about 5% of the population and that one or two copies of the allele will result in hyperhidrosis 25% of the time, whereas the normal allele will result in hyperhidrosis less than 1% of the time.We conclude that primary palmar hyperhidrosis is a hereditary disorder, with variable penetrance and no proof of sex-linked transmission. However, this does not exclude other possible causes, and we anticipate that genetic confirmation of this disorder may lead to earlier diagnoses and advances in medical and psychosocial interventions.