Medically called athelia, this is a rare condition but it is common in certain conditions. Athelia tends to occurs on one side (unilaterally) in children with the Poland sequence and on both sides (bilaterally) in certain types of ectodermal dysplasia.

The Poland sequence (named for Alfred Poland, a celebrated 19th-century British surgeon and ophthalmologist) is a unique pattern of one-sided malformations characterized by a defect of the chest (pectoralis) muscle on one side of the body and webbing of the fingers (cutaneous syndactyly) of the ipsilateral hand (the hand on the same side). It is right-sided three times more often than it is left-sided. The disorder is currently considered "a nonspecific developmental field defect" occurring at about the sixth week of fetal development. The cause is uncertain. In Poland syndrome there is aplasia of the sternal head of the pectoralis major: the end of the main chest muscle that normally attaches to the breastbone is missing. On that side of the body, nearby chest muscles (the serratus anterior and latissimus dorsi muscles) may also be absent as may be the armpit (axillary) hair. In girls, the breast on that side is also usually absent (amastia) and there is no nipple. There is athelia.

Ectodermal dysplasia is not one but a number of hereditary conditions usually characterized by the abnormal development of skin, absence of sweat glands, dry eyes and abnormal development of teeth -- all structures derived from the ectoderm (the outside layer in early embryonic development). The absence of sweat glands leads to an inability to sweat and heat intolerance.

Athelia also occurs in association with the progeria (premature aging) syndrome and the Yunis-Varon syndrome (a multiple congenital malformation first reported in 1980).

Athelia is distinguished from amastia, wherein the breast is absent, and from amazia, wherein breast tissue is absent but the nipple is still resent.