Dementia with Lewy-bodies (DLB) and Parkinson's disease dementia (PDD) are no rare causes of dementia. Both have neuropathologically, clinically, and neurochemically much in common. In the course of both conditions frequently psychotic symptoms occur, often induced by antiparkinsonian medication. Treatment of psychotic features with conventional antipsychotics is not tolerated in many cases. Therefore low-dose clozapine treatment is acknowledged usual practise for psychosis in Parkinson's disease and a case report indicates efficacy for psychosis in DLB, too. All other atypical antipsychotics except risperidone are not licensed for dementia in Germany, but risperidone is contraindicated in DLB due to manufacturer's notice and usually not well tolerated in DLB and Parkinson's disease. Open trials indicate safety for treatment of psychosis in DLB and PDD with quetiapine. Randomized controlled trials indicate, that quetiapine is less effective than clozapine against psychotic symptoms in both conditions, although comparatively safe. Cholinesterase inhibitors, especially rivastigmine, are a therapeutic alternative for treating both psychotic and cognitive symptoms in both conditions. Parkinsonism in DLB-patients responds worse to levodopa compared to patient with Parkinson's disease. Anticholinergic drugs often induce delirium in demented patients and therefore should be avoided. The same problem is associated with dopamine agonists in PDD and DLB. Amantadine, a NMDA-receptor antagonist like memantine, potentially bears the same risk of worsening psychotic symptoms. The following preliminary recommendation for drug treatment of PDD and DLB can be given: Stop all anticholinergic medication and reduce levodopa and other antiparkinsonian medication to the tolerated minimum. Levodopa alone is preferred. Treat with cholinesterase inhibitors to the maximum tolerated dose. If there is no adequate response regarding psychotic symptoms, add quetiapine. If this approach fails, replace quetiapine by low-dose clozapine. If behavioural disturbances are due to depression, anxiety, or irritability, treatment with an antidepressant, preferably citalopram, is an option.