Cavity ringdown spectroscopy (CRDS) of the hydroxyl radical (OH) has been explored in a laminar methane-air flame at atmospheric pressure over a range of equivalence ratio. Laser-induced fluorescence (LIF) of OH calibrated by CRDS in a lean flame compares well to PREMIX calculations using GRI-Mech 3.0. CRDS is a highly sensitive path-integrated diagnostic technique that can yield absolute absorber number densities via a relatively small number of measurable parameters. Among the chief advantages of CRIDS is that the measurement is independent of laser power and that the same laser used for LIF can be used for CRIDS with only a simple insertion of optics and detector for CRIDS. Since LIF has a large dynamic range and high spatial resolution, it is the preferred OH diagnostic for flame studies, however, it is difficult to directly quantify LIF measurements. For this reason, quantitative OH LIF measurements are normally obtained by calibration of the LIF signal using an independent technique. The utility of quantitative OH CRDS measurements for calibration of LIF in these flames is shown.