Objectives - The validity of memory and concentration complaints that are often reported after a whiplash trauma is controversial. The prevalence of malingering or underperformance in post-whiplash patients, and its impact on their cognitive test results were studied. Methods - The Amsterdam short term memory (ASTM) test, a recently developed malingering test, was used as well as a series of conventional memory and concentration tests. The study sample was a highly selected group of patients, who were examined either as part of a litigation procedure (n = 36) or in the normal routine of an outpatient clinic (n = 72). Results - The prevalence of underperformance, as defined by a positive score on the malingering test, was 61% (95% CI: 45-77) in the context of litigation, and 29% (95% CI: 18-40) in the outpatient clinic (p = 0.003. Furthermore, the scores on the memory and concentration test of malingering post-whiplash patients (n = 43) and non-malingering post-whiplash patients (n = 65) were compared with the scores of patients with closed head injury (n = 20) and normal controls (n = 46). The malingering post-whiplash patients scored as low as the patients with closed head injury on most tests. Conclusions - The prevalence of malingering or cognitive underperformance in late post-whiplash patients is substantial, particularly in litigation contexts. It is not warranted to explain the mild cognitive , disorders of whiplash patients in terms of brain damage, as some authors have done. The cognitive complaints of non-malingering post-whiplash patients are more likely a result of chronic pain, chronic fatigue, or depression.