The formation of carcinogenic Trihalomethanes (THMs) in the process of water disinfection by chlorine has raised concerns in the scientific community. This study aims to determine the levels of THMs in Greater Cairo drinking water over year of the study and compare it with the regulation in the Egypt law (458/2007), and the World Health Organization. The THMs concentration was measured in five different locations during 2014. The results indicate that the seasonal variation is below the critical values stipulated in the state, WHO and Egyptian regulations (seasonal average 45.14 ± 9.23 μg/l). In conclusion, it can be inferred that the concentration of THMs in the drinking water of Greater Cairo is not hazardous to human health.