Neurobehavioral changes observed in patients with brain tumours may appear as cognitive deficits, mood disturbances, changes in behaviour or decreased adaptability (e.g. drowsiness, apathy, loss of spontaneity, fatigue). They are initially subtle, develop insidiously, and their severity often changes. Serious diagnostic problems can be caused by mood disorders, psychotic symptoms, personality changes, from disinhibition to apathy, observed in such patients. The problem in distinguishing them from organic psychiatric disorders, often poses a challenge for psychiatrists, neurologists and general practitioners. We describe a case difficult to diagnose because of apathy, due to a brain tumour in the right frontal lobe, diagnosed as depression. Another difficulty, rather suggesting mood disorder, was rheumatoid arthritis. Thorough and meticulous analysis of clinical data, neuropsychological assessment and neuroimaging diagnosis may help to assess aetiology of the observed disorders which can have similar clinical pictures but various causes.