Heavy exposure to mercury in a vandalized dental office suite where 20 lb of mercury was maliciously spilled on floors (some covered with rugs) and splattered on walls during a nighttime break-in is described. Monitoring of ambient mercury vapor levels after a clean-up suggested that mercury contamination was under control, but urinary mercury levels that continued to increase contradicted these findings. A further investigation showed that some sections of rugs were heavily contaminated. Unaware of this contamination, dental assistants, by regularly vacuuming the rugs, created a hazardous environment. The steps taken to resolve this problem are discussed, and the importance of the urine analysis for the monitoring of mercury exposure is emphasized.