Diffuse hyperplastic goiter was diagnosed by histopathology in 11 perinatal bottlenose dolphins (Tursiops truncatus) that died at four separate zoos and aquaria. Thyroid morphology of these animals was compared with the histologically normal thyroids of two stranded wild bottlenose dolphin calves, a neonate and a 2-mo-old calf. Histologic changes included reduced follicular luminal diameter, markedly reduced or absent luminal colloid, hypertrophy of follicular epithelium, and follicular dysplasia. The etiology of the thyroid gland lesion was not identified. Cause of death was not determined for most of these animals, but they were presumed to have died from metabolic derangements associated with the thyroid lesion, drowning, or dystocia.