We report a unique case of a patient with a neuroendocrine tumor localized to the bone marrow. The patient had a history of hairy cell leukemia, and the neuroendocrine tumor was detected in a bone marrow biopsy specimen obtained to assess response to 2-chlorodeoxyadenosine therapy. The neuroendocrine tumor was present as nodules that replaced approximately 15% of the bone marrow medullary space and was composed of round cells with fine chromatin, indistinct nucleoli, and relatively abundant, granular, eosinophilic cytoplasm. Histochemical stains showed cytoplasmic reactivity with Grimelius and Fontana-Masson stains, and immunohistochemical studies showed positivity for keratin and chromogranin. The histologic, cytochemical, and immunohistochemical features resembled a carcinoid tumor, and metastasis to the bone marrow was considered initially. The patient was asymptomatic without diarrhea, flushing, or cardiac valve disease. Serotonin production, assessed by the measurement of serum 5-hydroxyindoleacetic acid and substance P levels, was normal. Extensive clinical and radiologic work-up and endoscopy of the gastrointestinal tract to detect a primary site other than the bone marrow were negative. Follow-up bone marrow biopsy 7 years after the initial diagnosis was positive for persistent neuroendocrine tumor. The patient has not received any therapy specific for the neuroendocrine tumor and has had no clinical symptoms or evidence of progression after 9 years of clinical follow-up. We suggest that this neuroendocrine tumor may have arisen in the bone marrow.