Although the benefits of relaxation are widely recognized, clinicians need to remain cognizant of its possible adverse effects. Equally, some people with mental illness may experience an exacerbation of their symptoms with the injudicious use of relaxation techniques that use imagery. This paper explores two conditions where vivid, often emotionally charged, images are problematic: (i) post traumatic stress disorder; and (ii) hallucinoses. Individuals who have these disorders may have difficulty controlling shifts between these internal and external events, or difficulty even recognising events as internal rather than external. Employing a relaxation technique that encourages imagery is likely to produce a stronger reaction because the impact of the imagery is increased by relaxation. This would result in an increase in arousal, the antithesis of relaxation. This paper suggests that relaxation techniques that avoid imagery or consider the patient's experience would be less likely to provoke unwanted reactions.