A pilot open study was conducted in order to evaluate the efficacy of clonidine in the treatment of LSD-induced hallucinogen persisting perception disorder (HPPD). Eight patients fulfilled entrance criteria. All complained of HPPD for at least 3 months and were drug free at least 3 months. They received fixed low doses of clonidine, 0.025 mg, three times a day for 2 months. They were evaluated by the Clinical Global Impression Scale (CGI) and a self-report scale on the severity of symptoms (graded 0-5). Patients scored an average of 5.25 (SD = 0.46) on the CGI and 4 on the self-report scale at baseline, indicating marked psychopathology. One patient dropped out at week 3 and a second patient dropped out at week 5. Of the six patients remaining at the end of 2 months, the average CGI score was 2.5 (SD = 0.55) and the self-report scale score was 2, indicating mild symptomatology. LSD-related flashbacks associated with excessive sympathetic nervous activity may be alleviated with clonidine in some patients.