We present the clinicopathologic characteristics of 110 colorectal mixed hyperplastic adenomatous polyps (MHAP) that exhibited the architectural but not the cytologic features of a hyperplastic polyp. They are compared with 60 traditional adenomas, 40 hyperplastic polyps, and five colonic polyps that contained admixed but well-defined hyperplastic and adenomatous glands (HP/AD). The patients with MHAP ranged in age from 15 to 88 years (mean, 63 years). Five patients had two or more (up to seven) lesions. MHAP measured 0.2-7.5 cm in diameter. They were distributed throughout the colorectum, but a slight preponderance of large lesions (more than 1.0 cm) occurred in the cecum and appendix. All MHAP were characterized by a serrated glandular pattern simulating that seen in hyperplasia (27% of MHAP were initially diagnosed as hyperplastic polyps). However, MHAP were distinguished by the presence of goblet cell immaturity, upper zone mitoses, prominence of nucleoli, and the absence of a thickened collagen table. Although surface mitotic activity, nuclear pseudostratification, and nuclear cytoplasmic ratio were greater in MHAP than in hyperplastic polyps, they were slightly less than in traditional adenomas. Thirty-seven percent of MHAP contained foci of significant dysplasia; 11% contained areas of intramucosal carcinoma. We conclude that these lesions reflect a morphologically unique variant of adenoma and suggest that they be termed "serrated adenoma" in order to emphasize their neoplastic nature. We further offer the hypothesis that MHAP may arise from the neoplastic transformation of a more differentiated cell in the crypt than the traditional adenoma.