Abstract: The present invention provides a method of determining whether a subject is afflicted with a depressive disorder comprising: (i) introducing into the subject a positron emission tomography (PET) radioligand capable of binding with a serotonin 5-HT1A receptor; (ii) performing one or more PET scans of the subject; (iii) determining, by analysis of the one or more PET images, a receptor binding potential of the PET radioligand for the serotonin 5-HT1A receptor in a region of interest in the subject; (iv) comparing the receptor binding potential value of the PET radioligand in the region of interest in the subject to a predetermined receptor binding potential threshold value; and (v) classifying the subject as having the depressive disorder or as not having the depressive disorder based on the comparison of step (iv), thereby determining whether the subject is afflicted with the depressive disorder.