['We assessed whether an activity data logger was able to detect and measure the reduced physical activity reported by women with moderate to severe primary dysmenorrhea. Twelve young women with a history of primary dysmenorrhea and 12 young women without a history of dysmenorrhea wore an activity data logger on their hip for 3 days when menstruating and for 3 matched days of the week when not menstruating. A visual analog scale was use to assess intensity of pain. When menstruating, the women with a history of primary dysmenorrhea, compared with when they were not menstruating, were significantly less active by about 40% on their day of worst pain (P < .001), day of intermediate pain (P < .001), and day of least pain (P < .001). There was no significant difference in the voluntary physical activity of the group on the 3 menstrual days. The women without a history of dysmenorrhea experienced mild menstrual pain but no significant decrease in physical activity (P = .82). We show that data loggers are able to detect and quantify the decrease in physical activity reported by the women with a history of moderate to severe dysmenorrhea and that menstrual pain but not menstruation itself was associated with decreased voluntary physical activity.'] Does actigraphy quantify reduced voluntary physical activity in women with primary dysmenorrhea?