We report a 35-year-old man with bilateral hydronephrosis found during a routine health examination. Kidney-ureter-bladder scout film showed a huge, oviform pelvic calculus (6.2 × 5.1 cm), and the calculus was treated endoscopically by electrohydraulic lithotripsy with excellent results. Bladder stones are a common problem, but it is rare for a calculus to be large enough to cause bilateral hydronephrosis in a young male. Surgical intervention by either cystolithotomy or endoscopic cystolithotripsy can achieve satisfactory results. Close follow-up is mandatory, because the recurrence of urolithiasis is high in those patients with voiding problems and there is also recurrent urinary infection. This case report illustrates the importance of sonographic evaluation of patients with bilateral hydronephrosis for the diagnosis of bladder calculi.