The variability in responsiveness to saturated fatty acids has not been studied systematically. For this reason data from three dietary studies carried out in our laboratory were pooled and used to evaluate how individuals vary in their responses in plasma concentrations of total cholesterol and low-density-lipoprotein cholesterol to the substitution of saturated fatty acids for unsaturated fatty acids. The data showed a marked variability in response. Some patients demonstrated a striking rise in cholesterol levels whereas others had more modest increases. This finding points to the need for further investigation on this issue.