F-18 FDG PET is used for the staging of many cancers. One of its limits is the analysis of the pelvis and the urinary tract because of physiological radiotracer excretion. We report a rare case of an 82-year-old woman in whom intravenous administration of diuretics (furosemide) allowed the identification of a primary diffuse large B-cell lymphoma of the bladder wall. This pharmaceutical increases urine flow and, therefore, reduces the concentration of radiotracer in the urinary tract, improving the visualization of pelvic or urinary tumors. Furosemide administration is an easy, safe, and noninvasive method, even in case of renal insufficiency. Newer PET cameras with high-performance crystals allow rapid acquisitions and improve the tolerance of an examination with diuretic injection. The 3-dimensional reconstruction of images in new PET cameras reduces the occurrence of hyperactive bladder artifacts. The present case illustrates the advantages of furosemide in a rare pathologic entity.