Dietary fatty acids have myriads of effects on human health and disease. Many of these effects are likely achieved by altering expression of genes. Several transcription factors have been shown to be responsive to fatty acids, including SREBP-1c, NF-kB, RXRs, LXRs, FXR, HNF4α, and PPARs. However, the relative importance of these transcription factors in regulation of gene expression by dietary fatty acids remains unclear. Here, we take advantage of a unique experimental design using synthetic triglycerides composed of one single fatty acid in combination with gene expression profiling to examine the acute effects of individual dietary fatty acids on hepatic gene expression in mice. The dietary interventions were performed in parallel in wild-type and PPARα-/- mice, enabling the determination of the specific contribution of PPARα. Depending on chain length and degree of saturation, dietary fatty acids caused a statistically significant change in expression of over 400 genes. Surprisingly, the far majority of genes regulated by dietary fatty acids in wild-type mice were unaltered in mice lacking PPARα, indicating PPARα-dependent regulation. We conclude that the effects of dietary fatty acids on hepatic gene expression are almost entirely mediated by PPARα, indicating that PPARα dominates fatty acid-dependent gene regulation in liver.