High-grade prostatic intraepithelial neoplasia (HGPIN) is the most likely precursor proliferation of peripheral zone, moderately to poorly differentiated prostatic adenocarcinomas. The usual cell type of the epithelial lining of HGPIN is a glandular epithelial cell with characteristic nuclear abnormalities. Here we report nine cases of unusual types of HGPIN, including three cases of signet-ring cell HGPIN, one case of small cell neuroendocrine HGPIN, and five cases of HGPIN with distinctive mucinous features. The three examples of signet-ring cell PIN were all associated with an invasive primary signet-ring cell carcinoma of the prostate. The HGPIN assumed a classical tufted and micropapillary architectural growth pattern, with the constituent cells exhibiting a morphologic appearance identical to that of the invasive signet-ring cells. The intraepithelial and invasive signet-ring cells were mucin negative and were immunoreactive for prostate-specific antigen (PSA). A fourth case displayed a mixed intraepithelial glandular-small cell neoplastic proliferation, where intraepithelial small cells were histologically identical to surrounding invasive small cell carcinoma cells. The small cell HGPIN and invasive small cell carcinoma cells were positive for the neuroendocrine markers chromogranin, synaptophysin, and neuron-specific enolase. In five cases, mucinous distension of HGPIN glands, producing a flat pattern of the epithelial lining layer, comprised the third unusual pattern of HGPIN. These blue mucinous secretions were readily detected by hematoxylin and eosin staining and were composed of both neutral (periodic acid-Schiff-positive) and acidic (alcian blue-positive) mucins. Herein we document the existence of an intraepithelial proliferation of neoplastic cell types-small cell neuroendocrine and signet-ring cell-that are usually considered as stromal-invasive cells in the prostate. The presence of these rare prostatic cell types in both HGPIN and invasive carcinoma provides further support for a close relationship between HGPIN and invasive carcinoma of the prostate. All three unusual types of HGPIN-signet-ring cell, small cell neuroendocrine, and mucinous-are important to diagnostically recognize because of the strength of association of HGPIN with invasive carcinoma.