This paper presents a literature review as well as a study in 13 patients suffering from delusion of infestation. Symptoms found in MMPI are related to clinical experience. Our results suggest the distinction of three different groups of these patients: 1. Patients with predominantly hypochondric traits, with a distinct conviction to suffer from a severe, physical illness that is often looked at as incurable (hypochondriacal delusion of parasitosis). 2. Patients with paranoid symptoms and without hypochondric apprehensions. These patients usually hopelessly fight against their parasites, that impair their sphere of existence. Physical illness is denied. ("delusion of infestation"). 3. Patients with signs of both, hypochondrical as well as paranoid delusions ("hypochondriacal delusion of infestation").