A phase II study of the efficacy and safety of FK506, a new potent immunosuppressant, has been conducted in 49 patients with GVHD after allogeneic BMT. Eighteen patients with acute GVHD and 31 with chronic GVHD entered the study. FK506 was administered at an initial dose of 0.05 mg/kg i.v. or 0.15 mg/kg orally twice a day to those whose GVHD had become uncontrollable with cyclosporin and/or other immunosuppressants. The response to FK506 was evaluated in 13 patients with acute and 26 with chronic GVHD. A marked response was observed in 5 and a good response in 2 of 13 patients with acute GVHD. For those with chronic GVHD, the response was marked in 2 patients, good in 10 and poor in 8. The most common adverse effects were renal toxicity (53.1%), followed by nausea and vomiting (30.6%) and a feeding of warmth (18.4%). There was a correlation between renal toxicity and whole blood levels of FK506. The dose should be adjusted to keep a trough level between 15 and 25 ng/ml. FK506 is promising in the treatment of both acute and chronic GVHD, even if it is intractable with other immunosuppressants, and may be most effective if administered early in the course of GVHD.