Important to the development and regulatory approval of therapies for XLHED will be the collection of data on the clinical history and prospective health of those affected by XLHED. The proposed natural history study will enroll male and female patients, ages 36 months and younger, who have a diagnosis of XLHED based on genetic testing and who have not received an investigational study drug. The study protocol will include collection of all relevant medical history and documentation of clinical outcomes using age-appropriate, minimally invasive technologies. Data will be collected both retrospectively, back to pregnancy assessments that may be available, and prospectively through age 5 yrs. Genotype-phenotype correlations in XLHED, based on well-documented health records and prospective assessments on genetically-confirmed individuals, may now provide new and clinically-predictive information for the benefit of patients, families, health care providers and clinical investigators designing trials for therapeutic interventions.