Pustular vasculitis is a new disease concept that links cutaneous, and possibly systemic, aspects of Behçet's, bowel bypass, bowel-associated dermatosis-arthritis, and disseminated gonorrhea syndromes. The pathomechanism of pustular vasculitic lesion generation may relate to circulating immune complex (CIC)-mediated vessel damage and serum enhancement of neutrophil migration. Thalidomide, an oral pharmaceutical available on strict protocol, has therapeutic effects based on proposed modulation of CIC- and neutrophil-mediated cytotoxicity. Thalidomide therapy was started for four patients with significant morbidity from Behçet's syndrome and for one patient with bowel-associated dermatosis-arthritis syndrome. Clinical benefit was dramatic in all patients who completed sequential four-week "on" and "off" thalidomide therapeutic cycles. In three of four patients, in vivo testing for CIC after histamine injection immunopathology converted from positive (immunoreactant deposition in dermal vasculature [four hours after histamine] and CIC-mediated vasculitis [24 hours after histamine]) to negative during therapy. No effects were noted on neutrophil migration or on the LFA-1/Mac-1/p150,95 family of glycoproteins associated with neutrophil adherence as assessed qualitatively by tritium labelling of neutrophil cell surfaces. In this small patient group, thalidomide was a clinically effective, safe (with rigid monitoring) therapy whose mechanism of action may relate more to inhibitory effects on CIC-induced vasculitis than to effects on neutrophil-mediated cytotoxicity.