Benzodiazepines are recommended for the treatment of Hallucinogen Persisting Perception Disorder (HPPD), although it is unclear which may be more helpful. Two out-patients with LSD-induced HPPD were successfully treated with clonazepam. They had not responded to low potency benzodiazepines or low doses of classic antipsychotics. After clonazepam discontinuation they reported a marked improvement and only mild symptomatology which persisted during a six month follow-up period. High potency benzodiazepines like clonazepam, which has serotonergic properties, may be superior to low-potency benzodiazepines in the treatment of some patients with LSD-induced HPPD.