Purpose: To evaluate whether valproic acid (VPA) treatment in children with epilepsy affects the plasma fatty acylcarnitine esters. Materials and Methods: Fifty children (mean age, 10±3.9) with epilepsy receiving VPA treatment for at least one year were evaluated along with 110 children of similar age as a control group. Levels of free carnitine and acylcarnitine profiles were studied with tandem mass spectrometry (Tan-dem-MS). Results: Free camitine levels were within the normal limits. Valproic acid was found to affect the levels of the following individual acylcarnitines (medians): C4-acylcarnitine (0.56 umol/L; controls 0.39 umol/L), 3-hyd-roxy-isovalerylcamitine (C5-OH) (0.23 umol/L; controls 0.14 umol/L), C14-acylcarnitine (0.11 umol/L; controls 0.16 umol/L), C16-acylcarnitine (0.76 umol/L; controls 2.5 umol/L), C16-OH-acylcarnitine (0.04 umol/L; controls 0.07umol/L), and Cl8:1-acylcarnitine (0.8 umol/L; controls 0.97 umol/L). A statistical comparison of these acylcarnitines between subjects and controls showed a statistically significant effect (p<0.05). Conclusion: The detection of normal free carnitine levels, the detection of 5-OH isovaleryl carnitine higher than other carnitines, and the normally found levels of some acylcarnitine fractions led us to conclude that VPA does not have a significant effect on the carnitine and acylcarnitine meta-, bolism as a drug side effect when the carnitine metabolism is considered. Therefore, we think that a study of the acylcarnitine profile in addition to the detection of the levels of free carnitine is essential in understanding the exact effect of VPA on the carnitine metabolism.