A randomized, blind study examined the effect of nail polish color on measurement of oxygen saturation by pulse oximetry. Fourteen adult volunteers had blue, green, purple, black, and red nail polish applied to their finger nails. A strip-chart recording of oxygen saturation (Nellcor N100) was made in room air and later interpreted in a blinded fashion. The absorption spectra of the five polishes were determined by spectrophotometry. The spectra of nine other nail polishes and three intravenous dyes also were examined. Black, blue, and green nail polish significantly lowered oximeter readings of oxygen saturation. Blue and green produced greater decreases than purple and red; black produced an intermediate decrease. Some but not all nail polishes absorbed light at the wavelengths used by the pulse oximeter (660 nm and 940 nm). The degree of artifactual desaturation correlated best with the difference between absorbance at 660 nm and absorbance at 940 nm (r = 0.95). Spectrophotometric absorbance data suggest that other colors may interfere with pulse oximetry. On the basis of spectrophotometric data, brown-red nail polish was predicted to interfere with oximetry; subsequent pulse oximetry measurements confirmed the prediction. Nail polish should be removed routinely before pulse oximetry monitoring.