It has been hypothesized that the ratio of heart rate variability in the low- (LF) and high- (HF) frequency bands may capture variation in cardiac sympathetic control. Here we tested the temporal stability of the LF/HF ratio in 24-h ambulatory recordings and compared this ratio to the preejection period (PEP), an established measure of cardiac sympathetic control. Good temporal stability was found across a period of 3.3 years (.46<r<.78), but the LF/HF ratio did not show the expected negative correlation to PEP, either between or within subjects. We conclude that the evidence to support the LF/HF ratio as a potential marker of cardiac sympathetic control in epidemiology-scaled research is currently insufficient. Copyright © 2008 Society for Psychophysiological Research.