To characterize the cytologic spectrum of small cell carcinoma of the urinary bladder, a review of 42 urinary cytology specimens from 13 patients with histologically proven tumors was conducted. Patients ages ranged from 45 to 81 yr (mean 68.9). In four tumors, small cell carcinoma was the sole malignant cellular component: all 11 urinary specimens in these patients harbored cells with features of an undifferentiated small cell carcinoma. In the remaining nine tumors, small cell carcinoma appeared with transitional, squamous cell or adenocarcinoma, but in four, small cell carcinoma was the sole invasive component. Almost one third of urinary specimens in this group lacked a small cell component. Neuroendocrine differentiation was confirmed by immunopathology in eleven cases (neuron specific enolase positive in 11 of 12, synaptophysin in 2/11, chromogranin in 2/13, Leu 7 in 2/7), and by ultrastructural analysis in two. Small cell carcinoma is a cytologically recognizable variant of bladder cancer, but admixture with other malignant components may mask its appearance in urinary specimens.