BACKGROUND Primary carcinoid tumor of the kidney is rare; only 18 patients have been reported. Because of the rarity of the lesion, its prognosis and clinicopathologic features are not well characterized, and its histogenesis is unknown. METHODS Clinicopathologic, immunohistochemical, and ultrastructural studies were performed on five patients with primary carcinoid tumor of the kidney. RESULTS Age of the patients ranged from 42 to 63 years, with a mean of 53.2 years. Only one patient had a carcinoid syndrome. Three were men, and two were women. In each patient, the tumor was solitary and confined to the kidney. All tumors had histologic features identical to those of carcinoid tumors affecting other sites. The cytoplasmic granules of the tumor cells were reactive with Grimelius stain in all patients. All tumors also were immunoreactive for keratin, neuron-specific enolase, and chromogranin, but only two patients had tumors that reacted for serotonin, pancreatic polypeptide, and vasoactive intestinal polypeptide. Ultrastructural studies showed numerous intracytoplasmic neuroendocrine granules in four tumors. One patient had Robson Stage I cancer, and two each had Stage II and IV cancer. The two patients with Stage IV tumor died of disease at 36 months and 28 months after diagnosis; one patient with Stage II carcinoid is alive without clinical evidence of residual disease 8 years after control of metastases; the other patient with Stage II and the patient with Stage I cancer are alive and well without evidence of disease at 36 months, and 2 months, respectively. CONCLUSIONS Tumor stage at presentation appears to be the most important factor in determining outcome. The number of mitoses and degree of cellular atypia appear to be important pathologic parameters in predicting prognosis, despite the limited number of patients reported in this report.