In 34 patients suffering from delusional parasitosis the relevance of an accurate differential diagnosis with respect to pharmacological treatment was investigated. Under a psychopharmacological therapy of the delusion's additional psychiatric symptomatology in 17 patients (50%) a full remission and in other 5 patients an improvement of the delusional symptoms was observed. The unexpected high recovery-rate is explained by the fact that all of the patients with additional depression showed a reduction also of the delusional symptomatology after a treatment with antidepressants. In contrast to that, patients with organic brain syndromes underlying the delusions had a significant worse outcome, which seems to be caused by the considerable lack of effective psychopharmacological treatment of such states. The results indicate that an accurate psychiatric differential diagnosis is of great importance for the psychopharmacological therapy of patients suffering from delusional parasitosis. In this context the Viennese decision-tree for delusional syndromes is discussed.