Sixty cases of uterine adenomatoid tumors (ATs) are reported. All except four were incidental findings in hysterectomy specimens, three of these being discovered preoperatively as large multicystic tumors. ATs were classified into two distinctive macroscopic patterns: small, solid tumors and large, cystic ones. The 56 small, solid ATs ranged from 0.2 to 3.5 cm, (average 2.1 cm); 48 were nodular and 8 diffuse. The four large, cystic tumors ranged from 7 to 10 cm. Inflammation occurred in 65% of the tumors, and a smooth muscle reaction, identified by an increased Ki-67 index, was present in most cases. Both types were histologically similar except for the presence of short papillae in cystic tumors, which also showed serosal involvement. Both were immunoreactive for cytokeratins, calretinin, HMBE-1, and vimentin. Estrogen and progesterone nuclear receptors and EMA were negative. These tumors represent a spectrum ranging from small and solid to large and cystic ATs in the female genital tract, whereas outside the genital tract they are morphologically similar to multicystic mesothelioma. Although a reactive origin for ATs often seems plausible, especially when inflammation is present, their neoplastic nature should not be ignored.