Skeletal cadmium (Cd) in corals is a promising proxy for oceanic upwelling. However neither a systematic evaluation of its natural variability, nor a calibration to environmental conditions has been made. Here, a high-precision, quantitative study of coral Cd using isotope dilution-inductively coupled plasma-mass spectrometry (ID-ICP-MS) is presented. Advantages of this approach over existing methods are: (1) reduced introduction of high-concentration elements into the MS, (2) decreased sample requirements (15 mg), and (3) improved precision of ±2.2% (for Cd/Ca) and ±2.3% (for [Cd]coral)