The misdiagnosis of functional disorders as other neurological conditions, Abstract Background Several studies have shown that when patients with functional neurological disorders are followed up, it is rare to find another neurological condition that better explains the initial symptoms in hindsight. No study has examined the reverse, studying patients with a range of neurological disease diagnoses with the aim of assessing how often a new diagnosis of functional disorder better explains the original symptoms. Methods A prospective multi-centre cohort study of 2637 new neurology outpatient referrals from primary care in Scotland. Neurologists provided initial diagnoses and a rating of the extent to which their symptoms were explained by an ‘organic’ neurological disease. Patients were followed up 19 months later with a questionnaire to their primary care physician asking about diagnostic change, and when indicated also by discussion with the original assessing neurologist and review of secondary care records. Results Valid responses were obtained for 2378 out of 2637 patients (90%) with symptoms ‘largely’ or ‘completely’ explained by organic disease at baseline. At follow-up, we found diagnostic errors in 48 patients. Of those, ten (0.4%) had a functional diagnosis and 38 patients (1.6%) had a different ‘organic’ diagnosis which better explained the original symptoms. Conclusions Patients diagnosed with neurological disease sometimes have a functional diagnosis at follow-up which, with hindsight, better explains the original symptoms. This occurs at a frequency similar to the misdiagnosis of ‘organic’ neurological disease as functional disorder. Misdiagnosis can harm patients in either direction, especially as we enter an era of evidence-based treatment for functional neurological disorders.