10 patients with frequent and severe bleeding from gastrointestinal vascular malformations took part in a double-blind, placebo-controlled, cross-over trial of a daily dose of 0.05 mg ethinyloestradiol plus 1 mg norethisterone given by mouth. Each arm of the trial lasted 6 months. Oestrogen-progesterone significantly decreased the transfusion need from 10.9 to 1.1 units packed cells (p less than 0.003). While on oestrogen-progesterone 2 of 9 patients required transfusions (mean 1.1 units packed cells per patient over 6 months), whereas all patients had to be transfused while on placebo (mean 10.9 units per patient over 6 months; p = 0.002 for number of patients). No significant excess of side-effects was noted with the active agents. The findings indicate that oestrogen-progesterone is an effective treatment for severely bleeding gastrointestinal vascular malformations.