This paper introduces yellowness index (YI) as a measure for chlorosis of leaves in stressed plants. YI provides a measure of the change in shape of the reflectance spectra between the maximum near 0.55 mu m and the minimum near 0.65 mu m. Quantitatively, YI is a simple, three-point approximation of the second derivative of the spectra, calculated using a finite divided difference approximation. YI is compared with two vegetation indices based on spectral changes in the region of the red edge, VI and NDVI. For manganese-deficient soybean leaves, YI, VI and NDVI were all closely related to leaf chlorophyll concentrations. YI is calculated from wavelengths in the visible that are thought to be less sensitive to changes in leaf structure or water content. Because YI is an approximation of a spectral second derivative, it should be less sensitive to atmospheric effects than many other vegetation indices. Finally, although the results reported here are promising, they are based on leaf-level observations and must be verified at the canopy level for remote sensing applications. An example of the correlation between NDVI and YI in AVIRIS data is given.