We undertook a detailed histologic study to identify specific morphologic features that may aid in distinguishing prostatic adenocarcinoma with lung metastases (PALM) from other pulmonary tumors with similar histologic features. In 16 cases, we found 3 predominant architectural patterns: microacinar (n = 10), tubulopapillary (ductal; n = 4), and carcinoid-like (n = 2). Characteristic features of PALM included small acinar and/or cribriform growth, frequent lymphangitic permeation, lack of stromal response, uniform round nuclei with prominent nucleoli, intraluminal blue mucin, and prominent cell borders. By immunohistochemical staining, prostate-specific antigen and prostate-specific acid phosphatase were present in 13 of 14 and 13 of 13 cases, respectively. Metastatic prostatic duct adenocarcinoma exhibited morphologic features similar to metastatic colonic adenocarcinoma. Two cases had a carcinoid-like appearance with nested or solid architecture, parachromatin clearing, and prominent nucleoli, but lacked the finely stippled chromatin pattern of carcinoid tumors. Several features that may result in misinterpretation or lack of association of the neoplasm in the lung with a prostatic primary include lung metastasis preceding the detection of a prostatic primary tumor, solitary pulmonary nodule, tubulopapillary (ductal) or carcinoid-like pattern, scant material in which histologic features of metastatic prostate carcinoma are not fully appreciated, and frequent necrosis. Attention to specific discriminating histologic features, supported by immunohistochemical staining, may be useful in the differential diagnosis, which is therapeutically and prognostically critical.