Background: Leiomyosarcomas of the urinary bladder (LMS-UB) are rare, usually aggressive neoplasms. Owing to their rarity, only a limited number of cases with clinical follow-up information have been published. There is no current consensus on LMS-UB grading, and it is unknown whether the widely accepted Fédération Nationale des Centres de Lutte Contre le Cancer (FNCLCC) and National Cancer Institute (NCI) grading systems of soft-tissue sarcomas are applicable to LMS-UB. Methods: The authors studied 34 well-characterised LMS-UB and compared the prognostic power of the FNCLCC and NCI systems with that of one published grading scheme for LMS-UB (Mayo). All available slides from 34 LMS-UB were retrieved and evaluated with regards to degree of differentiation, mitotic rate/10 high-powered fields (HPF), and % necrosis. Cases were graded using published criteria for the FNCLCC, NCI and Mayo schemes. Follow-up information was obtained. Results: The tumours occurred in 17 females and 17 males, ranging from 31 to 91 years (median 65), and measured 2-12 cm in size. One tumour was well differentiated, 17 tumours were moderately differentiated, and 16 tumours were poorly differentiated. Mitotic rates ranged from 1 to >30/10 HPF (median 12/10 HPF), and tumours showed 0-60% necrosis (median 25%). FNCLCC grades were 1 (3), 2 (12) and 3 (19). NCI grades were 1 (2), 2 (11) and 3 (21). Mayo grades were low (7) and high (27). FNCLCC and NCI grades were identical in 23/34 cases (68%). Four cases were FNCLCC/NCI grade 2 or 3 and Mayo low-grade. Clinical follow-up was available for 25 of 34 patients (74%). Clinical follow-up of ≥12 months was available for 17 of these 25 cases (68%) with a median follow-up duration of 52 months (range 12-120 months). Adverse outcome was seen in nine of these 17 patients (53%). Seven of the eight cases (88%) with a clinical follow-up duration of <12 months died of their disease. Overall, adverse outcome was documented in 16 of 25 (64%) cases. Metastatic disease was seen in 13 of 25 (52%) cases, with the lungs being the most common site of metastasis (62%). Adverse outcome was noted in 15 of 23 (65%) of FNCLCC grade 2 or 3 LMS-UB, as compared with zero of two (0%) FNCLCC grade 1 tumours (p=0.15), in 15 of 23 (65%) NCI grade 2 or 3 LMS-UB, versus zero of two (0%) NCI grade 1 sarcomas (p=0.17) and in 13 of 20 (65%) Mayo high grade LMS-UB, as opposed to two of five (40%) low-grade lesions (all results not statistically significant). Conclusions: The authors conclude that LMS-UB occurs in older adults of either sex and is characterised by aggressive behaviour, with adverse outcome in >60% of cases. Certain advantages of the FNCLCC system may support its more widespread adoption for future studies.