Foamy gland carcinoma is a variant of adenocarcinoma of the prostate that typically is assigned a Gleason score 3+3=6. The morphologic features of high foamy gland carcinoma have not been previously studied. We analyzed 55 cases of high-grade (Gleason score 7 or greater) foamy gland carcinoma of the prostate in needle biopsy (n=49) or transurethral resection (n=6) specimens. The number of cores involved by high-grade foamy gland carcinoma ranged from 1 to 12, with more than 1 core involved in 61% of cases (mean 3.4 cores). On average, 84% of the total tumor volume was foamy gland carcinoma, with high-grade foamy gland cancer averaging 73% of the total foamy gland carcinoma. The following results pertain only to the high-grade foamy gland cancer component. The most common architectural pattern was cribriform (73%), followed by fused/poorly defined glands (55%), cords/single cells (11%), and solid sheets (5%). Nuclear enlargement was observed in 45 of the 55 studied cases (82%). Prominent nucleoli were either absent or infrequent in 38 cases (69%). Frequent to numerous prominent nucleoli were seen more frequently in foamy gland carcinoma with Gleason score 8 or above (52%) than those with Gleason score 7 (16%) (P<0.004). Mitotic figures were observed in 22 cases (40%), and present in 65% of the cases with Gleason score 8 or above, but only in 22% of the cases with Gleason score 7 (P<0.002). In 31 cases (56%), intraluminal dense pink secretions were identified. Perineural invasion and extraprostatic extension identified on the biopsy specimens were noted in 18 cases (33%) and in 5 cases (9%), respectively. In 18 cases (33%), there was at least a moderate stromal reaction. A moderate or greater stromal reaction was seen in 48% (11/23) of the cases with Gleason score 8 or above compared with 22% (7/32) of the cases with Gleason score 7 (P=0.04). In 6 cases, there was a peculiar extensive desmoplastic reaction almost obscuring the carcinoma component, 5 of which were Gleason scores 4+4=8. Concurrent ordinary acinar nonfoamy adenocarcinoma was encountered in 26 of 55 cases (47%) with the following Gleason scores: Gleason 6 (27%); Gleason 7 (27%); and Gleason 8 to 10 (46%). Associated ordinary high-grade prostatic intraepithelial neoplasia and foamy gland variant of high-grade prostatic intraepithelial neoplasia/intraductal adenocarcinoma were seen in 13 cases (24%) and 11 cases (20%), respectively. Of the 19 cases with available immunohistochemical stains for high molecular weight cytokeratin, 7 (37%) showed nonspecific labeling of cancer cells in a nonbasal cell pattern. A similar finding was seen in 1 of the 7 (14%) cases with available stains for p63. α-methyl-CoA racemase positivity was noted in all 9 cases stained. In summary, uncommonly foamy gland carcinoma consists of cribriform, fused/poorly formed glands, cords/single cells, and solid sheets typical of Gleason patterns 4 and 5. High-grade foamy gland cancer shares certain morphologic features with more typical lower-grade foamy gland cancer including relatively bland nuclei with more difficult to identify nucleoli and frequent intraluminal dense pink secretions. However, consistent with their higher architectural grade, high-grade foamy gland cancers had more prominent nucleoli and increased mitotic figures compared with lower-grade foamy gland cancer. A unique subset of high-grade foamy gland carcinoma poses particularly difficult diagnostic challenges, with scattered, scant, relatively bland foamy glands imbedded in an extensive densely sclerotic desmoplastic stroma.