OBJECTIVES: To gather and compare clinical and histologic information from individuals affected by hypohidrotic ectodermal dysplasia (HED) and unaffected control subjects and to assess the value of these data in the diagnosis of HED. DESIGN: Volunteer subjects attending the 20th Annual Family Conference of the National Foundation for Ectodermal Dysplasia answered a questionnaire and performed a starch-iodide sweat-function test. A subset of the subjects also donated samples of hair and 4-mm punch biopsy specimens of palmar and scalp skin. Specimens from each of these tests were assessed in a blinded fashion. Analysis was performed comparing affected and control subjects for each of the following parameters: quantification of eccrine structures in the skin biopsy specimens, analysis of hair sample trichograms for hair shaft defects, and qualitative classification of starch-iodide palm-print sweat-function test results. SETTING: An international conference for families and individuals with ectodermal dysplasias. SUBJECTS: A total of 40 subjects were included in the final analysis: 15 unaffected control subjects and 25 subjects with HED. Nine affected subjects and 9 unaffected subjects donated skin biopsy specimens. MAIN OUTCOME MEASURE: This study was designed to assess the value of 4 simple tests in supporting the diagnosis of HED. RESULTS: Investigators were blinded to group during analysis of the test samples. Trichogram examination identified 3 hair shaft abnormalities, with a slightly higher prevalence in the affected group: variable shaft thickness, trichorrhexis nodosa, and pili torti. The sensitivity and specificity for each of these findings was less than 40%. Starch-iodide paper palm imprints identified a higher likelihood of diminished or absent sweat in the affected group, but this test had a low sensitivity (44%) and an imperfect specificity (93%). Examination of horizontally sectioned skin biopsy specimens from the palm were devoid of eccrine structures in a minority of affected subjects (sensitivity, 30%; specificity, 100%). In contrast, scalp biopsy specimens lacked eccrine structures in the majority of affected subjects (sensitivity, 67%; specificity, 100%). Separate analysis excluding the subjects without apparent eccrine apparatus yielded comparable numbers of eccrine ducts from control and affected groups. CONCLUSIONS: We have defined the value of simple, easily performed tests in the morphological diagnosis of HED. Noninvasive trichogram and sweat testing results can support the diagnosis of HED, but they are not sensitive or highly specific; horizontally sectioned 4-mm punch biopsy specimens of the scalp or palms that lack eccrine structures are diagnostic of HED; scalp biopsy is much more sensitive than palmar biopsy; and a scalp biopsy specimen with detectable eccrine structures suggests that a patient does not have HED.