5-Bromo-2'-deoxyuridine (BrdU) is frequently used to measure the turnover of cell populations in vivo. However, due to a lack of detailed mathematical models that describe the uptake and loss of BrdU in dividing cell populations, assessments of cell turnover kinetics have been largely qualitative rather than quantitative. In this study, we develop a mathematical framework for the analysis of BrdU-labeling experiments. We derive analytical expressions for the fraction of labeled cells within cell populations that are growing, declining, or at equilibrium. Fitting the analytical functions to data allows us to quantify the rates of cell proliferation and cell loss, as well as the rate of cell input from a source. We illustrate this for the BrdU labeling of T lymphocytes of uninfected and SIV-infected rhesus macaques.