Thyroid hyperplasia (goiter) has been considered a common problem in birds and is most commonly observed in budgerigars and pigeons. Records of the Zoo/Exotic Pathology Service (West Sacramento, CA, USA) were reviewed for the period from October 1984 to April 2001. From nearly 12 500 avian accessions, 30 reported a morphologic diagnosis of thyroid hyperplasia. Twenty-nine of 30 birds from varying species had multiple diagnoses at necropsy, while the remaining bird was diagnosed with thyroid hyperplasia alone. The appearance of all thyroid glands submitted was similar—the glands were enlarged bilaterally (approximately 2.7 × 1.4 cm in size) and red-brown or purple in color. Histologic changes to the thyroid parenchyma were diffuse in all cases (30/30). Thyroid glands contained numerous follicles lined by large cuboidal or low columnar epithelial cells. The morphologic diagnosis was thyroid follicular hyperplasia (hyperplastic goiter). Macaws were represented disproportionately (20/30), particularly blue and gold macaws (Ara ararauna), which represented 15/20 macaws. The cause of thyroid hyperplasia was not determined with certainty in the birds examined.