 This study found that the quality of the psychedelic experience, as measured by the altered states of consciousness questionnaire, was strongly correlated with long-term improvement in mental health. Specifically, the mystical type experience, as measured by the oceanic boundlessness dimension, was positively associated with long-term improvement in depression symptoms, while the anxiety-related dread of ego dissolution dimension was negatively associated with long-term improvement. These findings suggest that the quality of the psychedelic experience may be a key mediator of long-term change in mental health.