A circulating factor capable of inducing symptoms of dysmenorrhea has been demonstrated in 10 women by collecting plasma during periods of dysmenorrhea and infusing it during asymptomatic intervals. Typical symptoms of abdominal pain occurred in 8 of 12 women who received plasma infusion that had been drawn at the time symptoms were present. Plasma drawn at the time of no symptoms, when given to 10 women, produced no abdominal pain (P< .01). Combinations of pain and/or emotional irritability were found in 11 of 12 women after symptomatic units were infused and in only 2 of 10 women after asymptomatic units were infused (P<.001). The uterus need not be involved, as 7 of 10 patients had a hysterectomy prior to the infusions. It is suggested that a circulating factor may activate prostaglandin synthesis, which in turn may stimulate the neuronal system, thus causing symptoms.