Seasonal variations of trihalomethane (THM) concentrations were investigated within distribution systems of the Buyukcekmece water treatment plant in Istanbul City (Turkey). The investigation was based on an intensive 30-week sampling program, undertaken during the spring, summer and fall of the year 2003. THMs and other water quality and operational parameters were monitored at points along the distribution system between the treatment plant and the system's extremity. The results showed that THM concentrations vary significantly between finished waters and water at the distribution network. When water temperature exceeds 24 degrees C in summer, the THM levels are 1.2-1.8 times higher than finished water, while when water temperature is below 15 degrees C in the spring and fall, the measured THM concentrations at the system's extremity were very rarely higher than 100 mu g/L. Finally, THM levels were measured at sampling points representing progressively greater travel times from the plant to the extremity of the distribution system. Multiple regression analysis was also conducted in order to estimate THMs from total organic carbon, temperature, and chlorine dose parameters. The regression model resulted in a R-2 value of 0.827.