The effect of estrogen treatment in 31 patients with hereditary hemorrhagic telangiectasia (HHT) or Rendu-Osler-Weber disease was evaluated in a randomized double-blind trial. After a 3-month control period there was a 3-month treatment period during which 17 patients were treated with peroral estrogen (estradiol valerate), 4 mg dialy, and 14 with placebo. The results showed no significant reduction in the frequency of bleeding or its intensity. The hemoglobin values remained unchanged in the estrogen group compared with the previous control period as well as with the placebo group. The only demonstrable effect of estrogen treatment was a significant fall in transferrin.