Delusional parasitosis (DP) is a psychotic condition in which a person has the unshakeable and mistaken belief (delusion) and/or aberrant perception (hallucination) of being infested with parasites. The disorder will be usually classified in a primary DP-group without a detectable cause (so-called pure forms), while secondary DP-groups are associated with general organic conditions, psychiatric illnesses and drugs (substance induced). Etiology and pathophysiology of DP remain however unknown. In the present paper we hypothesize for the first time a decreased striatal dopamine transporter (DAT)-functioning (corresponding with an increased extracellular dopamine-level) as etiologic condition for DP (primary and secondary groups). The DAT as key regulator of the dopamine-reuptake in the human brain is well known (regulation of the extracellular dopamine concentration). It is a presynaptic plasma membrane protein highly dense represented in the striatum. The hypothesis of a decreased DAT-functioning as etiologic condition by DP is revealed in case reports which show that DAT-inhibitors, such as cocaine, pemoline, methylphenidate and other amphetamine-derivatives can induce the clinical expression of DP. Several other associated causes of secondary DP-groups (medications, parkinson, chorea huntington, multiple system atrophy, diabetes, cerebrovascular diseases, alcoholism, traumatic brain injury, hyperuricemia, human immunodeficiency virus, iron deficiency, schizophrenia, depression) suggest that the clinical expression of DP may be related to a decreased striatal DAT-functioning (blocking, reduced ligand binding, reduced density, reduced activity). Our examined DP-cases (2-females) show means of magnetic resonance imaging a structurally damaged striatum. Furthermore, we presume that by the primary DP-group, the physiologically age-related decline of the DAT-density is pathologically elevated. Based on this hypothesis we show in the present paper the relation between DP and decreased striatal DAT-functioning, trying to give a new insight into the pathophysiologically mechanism involved. The hypothesis provides supporting evidence that increased levels of extracellular dopamine in the striatum of DP-patients is likely to be the result of decreased DAT-functioning and not increased rates of release. The hypothesis can be investigated simply by dopamine transporter imaging in patients with DP.