We administered dipyridamole, an adenosine reuptake inhibitor, to 12 outpatients with DSM-III-R anxiety disorders (2 patients with generalized anxiety disorder, 10 patients with panic disorder). Dipyridamole was administered at a flexible dose in a single-blinded fashion following a placebo washout phase and elimination of placebo responders. The mean duration of active treatment with dipyridamole was 46 days (range 21-88 days); the mean peak dose of dipyridamole was 202 +/- 55 mg/day (range 100-300 mg/day). Symptom ratings were completed at regular intervals by the patient and by a research nurse unaware of the treatment condition. Clinically significant improvement in anxiety symptoms was not demonstrated. The implications of these findings for an adenosinergic dysfunction model of panic disorder are discussed.