Principal component analysis of ¹H NMR spectra of dichloromethane extracts taken from grapevine leaves reveals that phylloxera infestation of the root system causes metabolic changes in the leaves of infested grapevines, both in the field and in the glasshouse. A number of potential markers of phylloxera infestation were detected but their presence is transient and varies with the stage of the growing season. The changes in the metabolic profile caused by phylloxera infestation more closely resemble those caused by nitrogen deficiency than those induced by water stress. A reduction in the ratio of linoleic acid to linolenic acid in the triglyceride component of the leaf extract may provide an indicator of phylloxera infestation.