An exhaustive review on the organic illnesses presenting with psychiatric manifestations, properly defined pseudopsychiatric emergencies, is presented. A systematic classification of the numerous organic causes of psychiatric disorders, based on authors' experience and literature revision, is carefully analysed, and their suitable diagnostic management in emergency setting is proposed. Moreover, the role of bedside ultrasonography in Emergency Department is emphasized. The underlying pathogenetic mechanisms are separately discussed. A particular significance is given to "neuropsychological studies", displaying the complex connection between the central nervous system and the endocrine system. The role of immune system in influencing the central nervous system, explaining the model of "sickness behaviour" in inflammatory disease, is also described, according to recent reports of "psychoneuroimmunology". Moreover, the immune-mediated mechanism explaining how neoplasm can influence brain function in the "paraneoplastic syndromes" is shown. In order to facilitate the teaching method, organic illnesses presenting with acute psychic manifestations or mimicking specific psychiatric disorders are subdivided into three groups: (1) Endocrine and metabolic disorders and deficiency states; (2) Internal diseases; (3) Neurologic disorders.