Boys with X-linked hypohidrotic ectodermal dysplasia and their families were studied. Many suffered severe illness in early childhood and nearly 30% died; many had feeding problems, severe fever, atopic disease, and recurrent respiratory infections. Some infants failed to thrive. We found no consistent common endocrine or immunological abnormality, although, most had abnormal immunoglobulin production. This may be related to the abnormal mucosa of the gastrointestinal and respiratory tracts which exacerbates the chronic obstructive airways disease found later in life in those who smoke. Mental handicap was not a feature, although convulsions sometimes occurred during fever. Early diagnosis is important to avoid attacks of severe fever and so that rational management may be planned for other problems that arise. Dental advice should be sought before school age and genetic counselling may also be required. Many female carriers may be recognised at clinical examination: their affected sons can then be diagnosed more readily.