Functional weakness describes weakness which is both internally inconsistent and incongruent with any recognizable neurological disease. It may be diagnosed as a manifestation of conversion disorder or dissociative motor disorder. Other names include psychogenic or 'non-organic' paralysis. We aimed to describe the incidence, demographic and clinical characteristics of cases with functional weakness of less than 2 years duration, and to compare these with controls with weakness attributable to neurological disease. Both cases and controls were recruited from consultant neurologists in South East Scotland. Participating patients underwent detailed assessments which included: physical examination, structured psychiatric interview (Structured Clinical Interview for the Diagnostic and Statistical Manual of Mental Disorders), measures of symptoms, disability and distress [Short Form (36) Health Survey, Hospital and Anxiety Depression Scale], and assessment of their illness beliefs using an augmented version of the Illness Perception Questionnaire. In total, 107 cases (79% female, mean age 39 years, median duration of illness 9 months) were recruited. This number suggests a minimum annual incidence of 3.9/100 000. Forty-six controls (83% female, median age 39 years, duration 11 months) were also recruited. Compared to controls, cases had similar levels of disability but more physical symptoms, especially pain. They had a higher frequency of psychiatric disorders, especially current major depression (32 versus 7%, P < 0.0001), generalized anxiety disorder (21 versus 2%, P < 0.005), panic disorder (36 versus 13%, P < 0.001) and somatization disorder (27 versus 0%, P < 0001). There was no difference in median self-rated anxiety and depression scores. Paradoxically, they were less likely than controls to agree that stress was a possible cause of their illness (24 versus 56%, P < 0.001). Cases were twice as likely as controls to report that they were not working because of their symptoms (65 versus 33%, P < 0.0005). Functional weakness is a commonly encountered clinical problem. Patients with this symptom are as disabled as patients with weakness of similar duration due to neurological disease. There is a paradox between the frequency of depression and anxiety diagnoses and the patient's willingness to accept these as potentially relevant to their symptoms. We discuss the theoretical and practical implications of these findings for the concept of conversion disorder.