Drinking water of the Dammam metropolitan area is a blend of raw well water and desalinated water obtained from the Al-Azizia seawater desalination plant. Chlorination is the only disinfection method applied to the blended water before the water is pumped into the distribution system. Chlorination of water containing organic compounds results in the formation of trihalomethanes. In general, the known carcinogenic and physiological effects of THMs other than chloroform are limited. However, based on their structural similarity to chloroform, these compounds can be considered health hazards. The present study was carried out in order to determine the extent of the occurrence of THMs in finished drinking water in some of the major cities of the Eastern Province of Saudi Arabia.