It has not been possible to identify those low-grade papillary transitional cell bladder tumors that will recur based on conventional histopathologic assessment. Both the new World Health Organization/International Society of Urologic Pathology (WHO/ISUP) classification of transitional cell papillary neoplasms and the pattern of tumor cytokeratin 20 (CK20) immunostaining have been suggested as means of improving prognostication in low-grade transitional cell tumors. Forty-nine low-grade, noninvasive papillary transitional cell tumors were identified for the period between 1984 and 1993. The recently described WHO/ISUP classification was applied, and the tumors were classified histologically as papilloma, papillary neoplasm of low malignant potential (LMP) or low-grade papillary carcinoma. After CK20 immunostaining, the expression pattern in the tumor was classified as normal (superficial) or abnormal. Of 49 tumors, 20 were classified as papillary neoplasms of LMP and five of these patients (25%) experienced a recurrence. Of 29 tumors classified as low-grade papillary carcinoma, 14 (48.2%) recurred. In 46 of 49 cases, the CK20 immunostaining could be evaluated. Sixteen tumors showed normal (superficial) pattern of CK20 expression, and four (25%) of these patients experienced a recurrence. In contrast, of 30 patients with abnormal CK20 staining of their tumors, 15 (50%) patients had one or more recurrences. In this study, papillary neoplasms of LMP (as per the WHO/ISUP classification system) had a lower recurrence rate than low-grade papillary transitional cell carcinoma. Similarly low-grade urothelial tumors showing a normal CK20 expression pattern recurred less frequently than tumors with an abnormal pattern of CK20 staining. Neither of these differences was statistically significant, and recurrences were observed in 20% of patients whose tumors were both classified as papillary neoplasms of LMP and showed normal CK20 immunostaining; thus they do not allow a change in our current management of patients with low-grade papillary urothelial tumors, with close follow-up for all patients.