Trichloroacetic acid measured in first morning urine samples has recently shown promise as an exposure biomarker for by-products of disinfection. For trichloroacetic acid to be a valid biomarker of exposure it must be sufficiently persistent in the human body to encompass successive exposure intervals to disinfection by-products. The persistence of trichloroacetic acid in the human body was investigated by conducting an exposure-intervention study where participants consumed normal tap water for the exposure period and trichloroacetic acid-free bottled water for the intervention period. In an attempt to reduce intra- and inter-individual variability of trichloroacetic acid ingestion and excretion, ingestion sources were limited to water provided and ingestion amounts were managed. Entire first morning urine voids were collected from all participants for all days of the study and one participant collected all urine samples throughout each study day. Through determination of elimination half-life for trichloroacetic acid, it was found that trichloroacetic acid is persistent enough in the human body to facilitate its use as a biomarker of exposure (half-life range 2.1 - 6.3 d; R 2 range 0.76 - 0.94). It was also determined that first morning urine samples appear to be representative of total daily urinary excretion (average 0.38; range 0.18 - 0.55) and that half-life determination is the same whether first morning urine (half-life = 5.0 d, R2 = 0.80) or average of total daily urine (half-life = 5.0 d, R2 = 0.85) is used for the calculation. Together the results of this study indicate improved confidence in using trichloroacetic acid in urine as an exposure biomarker for disinfection by-products. Includes 21 references, figures.