Atypical cribriform lesions of the prostate (ACL) are cribriform glands lined by cytologically malignant cells with partial or complete basal cell lining. They represent cribriform high-grade PIN (cribriform HGPIN), which can be an isolated finding not associated with PCa (isolated ACL), or "intraductal carcinoma (IDC-P)" that is almost always associated with infiltrative high-grade prostate carcinoma (PCa) (cancer-associated ACL, ACL-PCa). We report the incidence, topographic relation to cancer, and morphologic differences of these 2 lesions in radical prostatectomy and discuss the potential biologic basis and implication for diagnosis in prostate biopsy. ACL was defined as cribriform glands comprising cytologically malignant cells that spanned the entire glandular lumens with partial or complete basal cell lining confirmed by basal cell immunostaining. ACL intermixed with, or within 3mm from the border of infiltrative PCa was categorized as ACL-PCa and was considered to be equivalent to IDC-P. ACLs other than ACL-PCa were considered as isolated ACL and equivalent to cribriform HGPIN. These histologic features of ACL were reviewed: number of ACL/prostate gland, size, glandular contour (round, irregular, branching), architectural pattern (dense cribriform, irregular cribriform, solid), comedonecrosis, and nuclear features (round and uniform, round with varying sizes, pleomorphic, giant nuclei [6× adjacent nuclei]). In 117 consecutive radical prostatectomy specimens, ACL-PCa and isolated ACLs were found in 21 (17.9%) and 15 (12.8%), respectively. ACL-PCa was more common in PCa with Gleason score more than or equal to 7 and higher tumor volume. The isolated ACLs were more common in Gleason score 6 PCa and their incidence was not significantly different among PCa with different tumor volumes. The mean number of ACL per prostate was 23.8 for ACL-PCa and 2.4 for isolated ACL (P=0.008). The size ranged from 0.2 to 9.0mm for ACL-PCa, and from 0.2 to 1mm for isolated ACL. The branching contour was present in 36/43 ACL-PCa, but only in 1/23 isolated ACL (P<0.001). The dense cribriform and solid architecture were present in 6 (14.0%) and 4 (9.3%) of ACL-PCa, but none of the isolated ACLs. Comedonecrosis was present in 14/43 (32.6%) ACL-PCa, and in none of the isolated ACL (P=0.001). The pleomorphic nuclei or giant nuclei at least 6X of the adjacent nuclei, were present in 12 (27.9%) ACL-PCa, but in none of the isolated ACL (P=0.005). ACL can be found both in association with PCa or without associated infiltrative PCa. Isolated ACL is uncommon, and the overwhelming majority of ACLs is associated with high grade (GS7) and high-volume PCa and represents IDC-P. Large gland size (>1mm), large focus involving many glands (number>6), complex architecture, and high-grade nuclei are characteristic of IDC-P. However, cribriform HGPIN and IDC-P overlap at the "low grade" architectural and morphologic spectrum and are difficult to distinguish based on morphologic criteria alone. If a biopsy contains a small number of ACL glands with "low grade" morphology, it should be diagnosed as "atypical cribriform lesion, cannot distinguish between cribriform HGPIN and IDC-P" and a repeat biopsy should be strongly recommended to rule out unsampled PCa. In contrast, if a biopsy contains ACL with one or several features of large focus, architectural complexity with large branching glands, pleomorphic or giant nuclei, or comedonecrosis, the biopsy should be diagnosed as IDC-P and definitive therapy should be recommended. © 2010 Lippincott Williams & Wilkins.