The effect of harvest period on fresh and dry leaf and root weights and ginsenoside contents of 2-, 3-, and 4-year-old american ginseng (Panax quinquefolium) plants was investigated. Ginseng plants harvested once every 4 weeks from the end of June through September had the highest and lowest fresh and dry leaf weights in June and September, respectively. The trend was reversed in roots, except for 3-year-old roots that exhibited maximum weight at the end of August. Total ginsenoside contents in leaves of 3- and 4-year-old plants increased with the growing season until the end of August, but in 2-year-old plants it increased until the end of September. Total ginsenoside contents in roots peaked at the end of June for 3- and 4-year-old plants.