The concentrations of the four commonly-identified trihalomethanes (THM; chloroform, bromodichloromethane, chlorodibromomethane and bromoform) in U.S. drinking water systems are regulated as a group. This report develops, applies and communicates a method to estimate internal exposures to these simultaneously-exposed chemicals. Because they are present in water used for drinking, bathing and other household uses, and because they are highly volatile, this work evaluated the development of internal doses via the oral, dermal and inhalation routes following residential exposures. This was accomplished by integrating several data sets that characterize human activity patterns, water use behavior, household volumes and ventilation, and THM concentration in drinking water. Physiologically based pharmacokinetic modeling was used to translate external exposures to internal doses for the simulated adult male and female and the 6-year-old child. Results indicated that inhalation exposures predominated and that children developed higher internal doses (mg/kg body weight) than adults in the same household. This report demonstrates the technical feasibility of combining stochastic and deterministic models and modeling approaches with 'real-world' concentrations of drinking water contaminants (here, THMs) to estimate internal doses for risk evaluation and for the examination of toxicokinetic interactions among mixtures of chemicals.