According to the rules of criminal procedure, medical evidence to prove the capacity to understand and give evidence may be requested when a witness is objected to on the ground of mental incapacity. This witness was an adult with learning difficulties whose competence was challenged and had been referred by the court for assessment in the course of the trial. The assessment instruments were the Wechsler Adult Intelligence Scale, British Adaptation (WAIS-R), a standard tool for the assessment of intelligence; the Gudjonsson Suggestibility Scale 2 (GSS2), which demonstrates suggestibility; and the Vineland Adaptive Behavior Scale, which assesses the extent of impairment of social functioning. Also, a set of structured questions was devised for this particular person. A total of 26 statements were compiled based on non-contentious issues relating to the participant’s life. These statements were presented in random order, and the witness was requested to respond “true” or “not true.” The participant’s scores on the intelligence test brought her within the category of someone with learning difficulties, however she was found to have a high level of social functioning. She was highly suggestible, but she showed she could distinguish between false and truthful statements, despite negative feedback. It was concluded that she would meet the requirements of a competent witness. It was also concluded that the approach of using a combination of standard psychometric testing and a series of structured questions was the most appropriate method of assessing witness competence. Intellectual tests and social function tests can be used in testing witness competence. Neither of these enables one to predict understanding of truthfulness. Structured questioning such as used in this case does allow such a prediction to be made. 34 references, appendix