We describe the case of a 4-year-old boy referred to our hospital with complaints caused by bladder stones. The patient was diagnosed with right-sided shrink kidney due to vesicoureteral reflux, urinary tract infections and a ureter stone seen 2 years previously. Now, four bladder stones were found and removed by high cystotomy and one was investigated by electron microprobe. We found a bladder stone made up of struvite and apatite with small particles up to 15 µm consisting of mercury and selenium, corresponding to the natural mineral tiemannite with the chemical formula HgSe. This is the first time that this mineral has been demonstrated in the human body by electron microprobe. The boy does not live in a mercury-polluted area nor was he eating relevant amounts of seafood contaminated with mercury. The most possible explanation of our findings is the intake of mercury and selenium by daily food and the elimination by urine. Precipitation of minerals may have been favored by urine alkalosis as a consequence of repeated urinary tract infections. The forming of tiemannite in the bladder stone can be interpreted as a natural detoxification process to eliminate mercury.