Solid and cystic ultimobranchial body remnants in the thyroid. In this study we determined the incidence rate (89%) and characterized the morphology of ultimobranchial body (UBB) remnants found in 18 serially sectioned neonatal thyroid glands. Although UBB remnants are often referred to as solid cell nests, we found cystic features in 55%. Ciliated columnar cells were seen in 23%. One contained a large pseudo-papilla. The UBB cells had nuclei with features reminiscent of papillary carcinoma nuclei in that they were enlarged, oval, and contained finely dispersed chromatin when compared with follicular cell nuclei. Both papillary carcinomas and UBB remnants are common, occur as tiny, solid, or cystic thyroid entities in patients of all ages, may contain papillary structures, and share some common nuclear features. Therefore, it is important to include UBB remnants in the differential diagnosis of minute thyroid entities and to recognize their morphologic features. 