Certain psychiatric complications are associated with various stages of PD. The possible causes known to date are analysed. Depression, isolated cognitive impairments, pharmacotoxic psychosis and dementia-related changes are the predominant mental disorders in PD. PD and depression syndrome occur very frequently in old age. Behaviour and mimicry of patients with progressive PD and of patients with depression syndrome are sometimes so similar that the two conditions can be differentiated only by long-term monitoring. In addition, PD and depression may occur simultaneously. However, frequency and intensity of depressive phases do not differ in PD patients and aged-matched depressed patients without PD. About one third of patients hospitalized at the neurological department of the Geriatric Hospital Lainz require antidepressant drug treatment. Similar percentages were found for other chronic cerebral and extracerebral diseases in the aged. Major depressions are independent of the parkinsonian disability and can be successfully managed only by antidepressant medication. Pharmacotoxic psychoses are not only serious conditions, they also reveal the limitations of therapeutic options. The unusual frequency of such acute psychoses, i.e. 30 to 60% in the terminal stages of the disease, indicates a special relation between antiparkinson medication and increasing neurotransmitter disturbances. Permanent pronounced depression in the sense of DSM III is not one of the symptoms of typical PD. States of dementia occur only in connection with a second or third cerebral pathology, mostly in combination with SDAT and MID.