In the past years, there has been growing awareness about childhood onset multiple sclerosis (MS) and the relevance of psychosocial aspects such as cognitive disturbances, fatigue and depression in this population. We describe a case of a 16-year-old patient with relapsing-remitting multiple sclerosis (RRMS) who presented at our clinic with severe fatigue symptoms and who underwent repeated neuropsychological examinations. A sudden significant slowing indicated a new relapse while neurological examination did not. This case highlights the high sensitivity and clinical relevance of neuropsychological testing in patients with juvenile MS even in the context of treatment decisions.