Patients with primary or recurrent low/intermediate-risk papillary urothelial carcinoma of the bladder will undergo resection of all but one marker lesion, measuring at least 6mm but no greater than 10mm, and biopsy of normal-appearing mucosa. Patients with a solitary tumor will undergo only biopsy prior to treatment. A 2mm cold cup biopsy of the marker lesion will always be performed to rule out a potential high-grade lesion and for assessment of pretreatment immunohistochemistry expression levels of ERα, ERβ1, Ki-67 (proliferation marker) and TUNEL (apoptosis marker). If there are multiple tumors, all lesions, except the marker lesion will be resected and sent for analysis. These patients will not receive single immediate post-operative intravesical instillation of mitomycin-C. They will then undergo a 12-week course of treatment with tamoxifen administered as a single daily oral dose of 20mg. At the completion of therapy, patients will undergo resection of the marker lesion (or biopsy of the tumor bed, if a complete response is observed) and biopsy of normal-appearing bladder mucosa again. Toxicity evaluations will be performed at the beginning (day 3), midway (week 6), and at completion of treatment (week 12), prior to resection of the marker lesion. A final assessment for toxicity will also be performed 30 days after completion of therapy as well as a second definitive resection of the marker lesion. Urine samples will be obtained with the index tumor in place (marker lesion), and after completion of treatment, at the time of definitive transurethral resection of the index tumor, as part of the standard clinical care of these patients, and at the discretion of the surgeon for assessment of urinary cytology. The urine samples will not be utilized for the research study. All normal-appearing bladder biopsies (pre and post-treatment), the additional tumors (in case of multiple lesions), and the definitive resection of the marker lesion (in the absence of response to therapy) will provide sufficient material for immunohistochemistry assessment of the expression levels of ERα, ERβ-1, ERβ-2, ERβ-5, Ki-67, and TUNEL, and also for RT-qPCR for mRNA analyses of ERα, ERβ-1, ERβ-2, and ERβ-5. The pretreatment biopsy of the marker lesion will be performed with a smaller biopsy forceps and will provide limited material, sufficient only for immunohistochemistry assessment of the expression levels of ERα, ERβ-1, Ki-67, and TUNEL.