Kudzu (Pueraria lobata) is widely considered a weed in the USA. However, the roots and flowers of kudzu are commercially available as crude Oriental medicines. Since the leaves of kudzu contain similar phytochemicals, kudzu leaves seem to be a promising health food or herbal supplement. Therefore, we investigated the variation in amounts of phytochemicals in kudzu leaves during growth using a high performance liquid chromatographcoupled to an evaporative light scattering detector. The amount of total saponin obtained from collected kudzu leaves from May to January was fairly consistent (0.10%-0.19%). In contrast, the amount of total flavonoid varied from 0.78% (June 12) to 2.21% (October 23). The increase in content of flavonol glycosides was larger than that of isoflavone glycosides. Interestingly, the amount of quercetin glycosides gradually increased from May to November, whereas the amount of kaempferolglycosides did not change much. The total amount of phytochemicals increased until late autumn. Therefore, the most appropriate time to harvest the leaves would be late autumn, just before the leaves wither.