A new scientific report associates estrogen therapy with an increased risk of developing a blood clot in a vein (venous thrombosis) in postmenopausal women who went through hysterectomy. The risk appears to be smaller than the combination of estrogen plus progesterone given to women who still have their uterus, not undergoing hysterectomy, the procedure through which the uterus is removed. Researchers analyzed data from 10,739 women, between 50 and 79 years of age, who went through hysterectomy in the past and received either estrogen or placebo. During the 7 years in which they were followed, 111 women who received estrogen developed a blood clot as opposed to 86 women who received placebo. Venous thrombosis affects about one adult per 1,000 and includes deep vein thrombosis, blood clots in a deep vein and pulmonary embolism, a life-threatening illness in which the blood clots move to the lungs. However, there was little information about the effects of estrogen alone. "There was no certain evidence before that estrogen alone caused these events, so some people had been promoting just estrogen," Dr. J. David Curb, professor at the University of Hawaii. The risks of venous thrombosis and of pulmonary embolism were slightly higher for women who took estrogen alone, while the risk of developing deep vein thrombosis was quite higher. "Women have to be very cautious about taking estrogens. There may be certain women that need them for severe symptoms, but there are just multiple risks and while each one of them presents a relatively small risk for a person, they all add up," said Curb.