The case history of a woman with occlusive hydrocephalus caused by a fourth ventricle cyst is presented. She had slowly progressive complaints and symptoms--concentration and memory disturbances, low-tempered mood, then slight dizziness, loss of appetite and progressive headache--transitorily misinterpreted for signs of depression. She had been treated by psychotherapy and antidepressants for months. Since she did not improve she was referred to a psychiatric hospital. The rapidly progressing neurological syndrome with worsening headache, gait disturbance and vomiting was finally identified and it turned out to be caused by a fourth ventricle CSF blockage of unknown aetiology. An acute neurosurgical intervention was indicated. It revealed a huge fourth ventricle cyst, undetectable on MRI, occupying the whole ventricle. The resection of its walls resulted in complete recovery. We conclude that since unspecific mental complaints and symptoms suggesting depression may be misleading, their organic origin has to be excluded.