Breast reduction surgery or mastectomy following administration of androgen therapy is part of the female-to-male gender reassignment process. Details regarding the histopathologic findings in breast tissue from patients undergoing female-to-male gender reassignment surgery are limited. We reviewed hematoxylin and eosin-stained sections of breast tissue from 148 patients who underwent breast reduction surgery or mastectomy as part of the female-to-male gender reassignment process at our institution between January 2014 and May 2017. The spectrum of histologic features in each case was catalogued. The median patient age was 27 years (range 18-60 years). Lobular atrophy was seen to some degree in 73% of cases and was prominent in 42%. A predominantly fibrotic stroma was seen in 45% of cases and areas resembling the fibrous stage of gynecomastia were seen in 41%. Other features included variably ectatic ducts in 96% of cases, cysts in 42%, apocrine metaplasia in 32%, fibroadenomatous change in 27%, usual ductal hyperplasia in 26%, and pseudoangiomatous stromal hyperplasia in 19%. Five cases (3%) demonstrated atypical hyperplasia (atypical ductal hyperplasia in 2; atypical lobular hyperplasia in 2; both atypical ductal hyperplasia and atypical lobular hyperplasia in 1). One case demonstrated high grade ductal carcinoma in situ. No invasive carcinomas were identified. In conclusion, the majority of breast specimens from patients undergoing female-to-male gender reassignment demonstrate at least some degree of lobular atrophy as well as ectatic ducts, fibrous stroma, and areas resembling the fibrous stage of gynecomastia. Only rare cases showed atypical lesions; the clinical significance of these lesions in this setting is uncertain.