Background. Primary carcinoid tumor of the kidney is rare; only 18 pat
ients have been reported. Because of the rarity of the lesion, its pro
gnosis and clinicopathologic features are not well characterized, and
its histogenesis is unknown. Methods. Clinicopathologic, immunohistoch
emical, and ultrastructural studies were performed on five patients wi
th 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 patie
nt had a carcinoid syndrome. Three were men, and two were women. In ea
ch patient, the tumor was solitary and confined to the kidney. All tum
ors had histologic features identical to those of carcinoid tumors aff
ecting other sites. The cytoplasmic granules of the tumor cells were r
eactive with Grimelius stain in all patients. Alt tumors also were imm
unoreactive for keratin, neuron-specific enolase, and chromogranin, bu
t only two patients had tumors that reacted for serotonin, pancreatic
polypeptide, and vasoactive intestinal polypeptide. Ultrastructural st
udies 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 diseas
e at 36 months and 28 months after diagnosis; one patient with Stage I
I carcinoid is alive without clinical evidence of residual disease 8 y
ears 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. Conclusion. Tumor s
tage at presentation appears to be the most important factor in determ
ining outcome. The number of mitoses and degree of cellular atypia app
ear to be important pathologic parameters in predicting prognosis, des
pite the limited number of patients reported in this report.