Two patients with fibrodysplasia ossificans progressiva (FOP) were treated with disodium etidronate, a disodium salt of ethane-1-hydroxy-1, 1-diphosphonic acid (EHPD) for two years; both had multiple ectopic ossifications of connective tissue. Regular follow-up showed no improvement in joint movement whilst laboratory examination revealed an increase in the serum phosphorus level. Serum calcium and urinary phosphorus excretion remained within normal limits. The urinary calcium excretion varied with the dose of EHDP. X-ray examination showed significant reduction in size and density of the ectopic ossifications, while the density of bones during and after EHDP therapy remained normal. Development of new ossification was inhibited in the active phase of the disease. Suspected side-effects of the drug were occult blood in stools, skin petechiae, and amenorrhoea. Previous reports on the EHDP treatment of disorders with ectopic ossification are summarised.