Abstract Introduction Peutz–Jeghers syndrome is an autosomal dominant disease with incomplete penetrance and variable expression caused by germline mutation of serine threonine kinase 11/liver kinase B1; it is characterized by hamartomatous polyps in the gastrointestinal tract, mucocutaneous melanin pigmentation, and increased predisposition to neoplasms. In Peutz–Jeghers syndrome, bilateral Sertoli cell testicular tumors cause endocrine manifestations including gynecomastia and feminization. This study aimed to assess the role of breast ultrasound in the evaluation of the effectiveness of an innovative surgical approach. Case presentation This report presents a pair of European 9-year-old identical male twins with Peutz–Jeghers syndrome, bilateral prepubertal gynecomastia, and testicular multifocal calcifications. Both twins were treated with anastrozole for 2 years. After finishing treatment, both underwent subcutaneous mastectomy performed by the “modified” Webster technique. Breast examination and ultrasound were performed before and after the pharmacological and surgical treatment. A breast ultrasound scan before surgery showed bilateral gynecomastia in both patients. No solid nodular or cystic formations were present on either side. After pharmacological therapy and surgical glandular removal, a breast examination showed a significant reduction in breast volume; 1 year after surgery, a breast ultrasound scan of both patients showed a total absence of glandular parenchyma, with muscle planes well represented. Conclusions Breast examination and ultrasound have proved to be a valid approach in the assessment of the treatment of prepubertal gynecomastia because they allow the efficacy of the pharmacological and surgical treatment to be evaluated in a multidisciplinary approach to one of the most frequent endocrine manifestations of Peutz–Jeghers syndrome.