Background “Effort tests” (i.e., tests that indicate whether appropriate effort is being made) are increasingly used in neuropsychological assessments. However, not all failures on effort tests are necessarily attributable to lack of effort. Understanding the factors that contribute to failure on effort tests has important clinical implications. Objectives This study aimed (a) to establish the extent to which patients with medically unexplained symptoms (MUS) presenting to neurology fail effort tests, (b) examine the performance of “mild” and “strong” simulators on effort tests, and (c) compare the performance of simulators and MUS patients. Methods Forty-three MUS patients, 35 mild simulators, and 39 strong simulators undertook a battery of effort tests. The groups were matched for age, estimated IQ, and gender. The MUS patients were not involved in litigation. Results The failure rate (defined as failing two or more effort tests) was 11% in the MUS group, 94% in the mild simulation groups, and 100% in the strong simulation group. Conclusions A minority of nonlitigant MUS patients who present to neurology fail effort tests. Possible reasons for this are discussed. In that they do not produce below-chance performance (in contrast with many of the simulators), there is no clear indication of deliberate failure, though this cannot be ruled out. The very high failure rate in the mild simulation group (who were instructed not to overact) indicates the sensitivity of effort tests to lack of effort and the value of including such tests in neuropsychological assessments.