Clinically based diagnostic criteria for DLB have limited accuracy. The availability of a biomarker to assist with diagnosis would be a major advance. Severe nigro-striatal degeneration and dopamine loss occurs in DLB but not in most other dementia subtypes offering a potential system for a biological marker. In the PDT-301 study, 326 patients with dementia with clinical diagnoses of probable or possible DLB, or non-DLB dementia established by a Consensus panel, had a FP-CIT SPECT brain scan labelling the dopamine transporter (DAT) reuptake site in the striatum. Three readers, blinded to clinical diagnosis, classified the images as normal or abnormal by visual inspection. This study which was conducted across 40 European sites, confirms the high correlation between abnormal (low uptake) DAT activity measured using FP-CIT SPECT and a clinical diagnosis of probable DLB. The diagnostic accuracy is sufficiently high for this to be clinically useful in distinguishing DLB from AD.