In this case of a 54-year-old woman, severe impairment of motor and sensory function - that could not be assigned to any neurological disease - was diagnosed as a probable conversion disorder or functional neurological disorder (FND). Several psychological stressors, which occurred in the year that preceded the first development of symptoms, were linked to the disorder. Nerve conduction values had not shown any abnormalities at the onset of the disease. However, as the condition progressed, cerebrospinal fluid and nerve conduction study proved an underlying polyneuropathy. In this article we discuss the diagnostic process followed in this case. General lack of evidence for psychological conflict being related to symptoms of conversion disorder/FND led to elimination of this criterion in DSM 5. Instead, the diagnostic process of conversion disorder/FND requires not only exclusion of neurological causes for the symptoms, but also active examination of neurological findings that are discrepant with known neurological diseases; taken together this can positively support a diagnosis of conversion disorder/FND.