L1 cell adhesion molecule (CD171) represents a strongly unfavorable prognostic biomarker for ovarian and endometrial carcinomas. Here we carried out an immunohistochemical survey of L1 expression in normal adults and in a broad range of benign and malignant tumors using monoclonal antibody L1-11A and the novel monoclonal antibody L1-14.10. In normal tissues, L1 was expressed in the collecting tubules of adult tissues and pediatric kidney and in peripheral nerve bundles. In tumors of the female genital tract, L1 was detected in adenocarcinomas of the cervix and fallopian tubes, in addition to ovarian and endometrial carcinomas. Nongynecological tumors expressing L1 comprised malignant melanoma, colon adenocarcinoma positive to chromogranin, clear-cell adenocarcinoma of the urinary bladder, pheochromocytoma, small cell lung carcinoma, and tumors of the nervous system. L1 was absent in breast carcinoma, gastrointestinal tract carcinomas, gastrointestinal carcinoids, renal clear-cell carcinomas, prostate adenocarcinomas, and mesotheliomas. Surprisingly, L1 expression in established breast and renal carcinoma cell lines was not a predictor for its presence in these human tumors in vivo. Our results suggest that L1 expression in tumors is not ubiquitous but restricted to certain subtypes and may be a helpful molecular marker for differential diagnosis and target for antibody-based therapy.