Drug-induced psychiatric states occur frequently in PD. In the prelevodopa era, depression and other psychiatric disorders were described in PD, but in untreated patients psychosis was rare. Since the development of levodopa and other pharmacological treatments for PD, however, psychotic symptoms have become much more common (10-50%). In some individuals these problems can be more disabling than the motor features of PD and, as a result, pose a serious threat to the patient's ability to maintain independence. The drug-induced psychoses consist of several distinct psychiatric syndromes that can be divided broadly into those occurring on a background of a clear sensorium and those which are accompanied by confusion and clouding of consciousness. Benign organic hallucinosis is the most common of these syndromes (30%). It usually occurs on a background of a clear sensorium and may not be a particularly troublesome problem if the patient is able to retain insight into the nature of these symptoms. More disabling syndromes usually include delusional thinking that is frequently paranoid, confusion and even frank delirium. Although all these psychotic syndromes can occur in isolation, there is a tendency for mild symptoms to progress to more disabling ones if adequate and timely treatment is not instituted. Abnormal dreaming and sleep disruption often precede these difficulties by weeks to months and may provide an important early clue to their onset. The mechanisms responsible for drug-induced psychotic symptoms in PD are unknown, but dopaminergic (especially mesolimbic) and serotoninergic systems are likely to be involved. The treatment of the drug-induced psychoses in PD should be undertaken in a stepwise manner. A detailed discussion of this approach, including the use of anti-PD medication adjustment, clozapine, and other medications (neuroleptic and nonneuroleptic) and ECT is provided (see Fig. 1). Although drug-induced psychoses are the most important of the drug-induced psychiatric states, mania, anxiety, and hypersexuality may also occur. Depression is also common in PD, but it is unlikely to occur as a side effect of antiparkinsonian medications.