A 29-year-old man presented with progressive behavioural changes, expressed mainly in the form of increasing of apathy and self-neglect. The initial differential diagnosis included a psychotic disorder, a mood disorder or a personality disorder. After a month of medication-free observation and uncertainty regarding the results of various diagnostic approaches, we decided to use MR-imaging; this revealed a frontal meningioma which had invaded the entire frontal lobe. Although patients with some types of somatic disorders frequently present with psychiatric symptoms, there are often indications (e.g. from history or physical examination) that in fact the symptoms are of organic origin. In this case report we discuss the indications that should lead clinicians and psychiatrists to consider the possibility of organic pathology when young adults present with only psychiatric symptoms.