The authors report a case of a 61-year-old woman diagnosed with large bladder diverticulum. Diagnosis was performed only after a series of investigations carried out for the occasional finding of hypercreatininaemia. Although the significant volumes of post void residual (PVR) and the relevant urine stagnation in the diverticulum, subjective symptomatology was absent and urinalysis and urine culture were negative. The scheduled therapeutic plan consisted of fosfomycin three grams every ten days for six months, self-catheterization twice a day, voiding on a time schedule, and adequate fluid intake. The monthly scheduled follow-up at one year showed good general health, good compliance with the therapy, no urinary tract infections, a decrease in creatininemia to 1.2 mg/dl, and regression of nephrohydrosis to a mild stage. In conclusion, the absence of symptoms and negative urinalysis or urine culture allows expectant management despite the considerable size of the bladder diverticulum.