Polythelia within Becker's naevus.

We report a case of accessory mammary tissue of type 2a (or polythelia) entirely localized within a Becker's naevus of mixed type in a 32-year-old Caucasian woman. Polythelia was congenital in origin while Becker's naevus appeared during puberty. Renal ultrasonographic studies, laboratory examinations and other instrumental investigations were either normal or negative. Although the association of accessory mammary tissue with Becker's naevus has previously been reported--also in the form of mammorenal and acromammorenal syndromes--this is the first case characterized by an anatomic overlap between the two anomalies. It may be the expression of a common disturbance acting early in embryogenesis in the specific developmental fields involved.