In a pilot study, 26 patients with acute completed strokes (48 hours to 5 days after cerebral infarction) were randomly assigned to the prostacyclin (PGI2) or placebo groups. PGI2 sodium salt (Epoprostenol, Wellcome Research Laboratories and Upjohn Company) or its solvent (glycine buffer) were infused into the subclavian vein for six-hour periods in five courses separated by six-hour intervals. Prostacyclin was administered at a rate of 2.5-5.0 ng/kg/min. A significant alleviation of neurological deficits occurred 6 and 54 hours after the treatment in patients receiving prostacyclin. This improvement lost its statistical significance at the end of a two-week observation period. It is concluded that further modified controlled studies are required to evaluate the therapeutic usefulness of PGI2 in the treatment of patients with cerebral ischaemia.