In criminal trials, the credibility of witnesses is determined by their ability to tell the truth as well as their motivation to tell the truth. However, even when witnesses are sufficiently able and motivated to tell the truth, the accuracy and reliability of their statements may be influenced by psychological vulnerabilities and questioning styles. This is particularly the case when witnesses are part of vulnerable groups, such as people with learning disabilities. Previous research has revealed that suggestibility is a key factor in explaining why some witnesses make false statements regarding crimes for which they are not responsible; this is a particular challenge when interviewing individuals with learning disabilities. The technique of statement validity analysis has been used to discriminate truthful from fabricated witness accounts involving allegations of sexual abuse of children. The author presents a case study using statement validity analysis to examine the testimony offered by Ms. X, an adult suffering from learning difficulties. The potentially significant problems that emerged in the interview with Ms. X are analyzed, including problems encountered during the rapport stage, in the timing of questions, and in the questioning style. An analysis is also offered of the interview with the defendant in the case, Mr. Y, which focuses on an assessment of his understanding of the questions posed to him and the reliability of his account of the events, including an apparent admission of guilt. Specific examples of problematic questioning are presented. References, appendix