After a presentation of the traditional principles of diagnosis and a precise definition of terms, the Vienna approach to the classification of delusional conditions is introduced. This approach is multiaxial: the first axis consists of a classification of delusional syndromes based on a cross-sectional description of their structural and constituting elements; on the second axis, the relationship between the delusional world and the real one is described; the third axis is for the recording of delusional contents; the fourth axis serves for an attempt at etiological attribution. The Vienna approach differs principally from the usual systems of classification in that it is purely syndromatological and thus it avoids giving an a priori nosological meaning to the various symptoms making up the delusion (for example, certain delusional themes, certain forms of hallucinations). A definitive nosological diagnosis is only possible when an organic cause is clearly evident, otherwise the suffix '-morphic' (endogenomorphic-schizophrenic, endogenomorphic-cyclothymic, and organomorphic axial syndromes) is added to denote the close resemblance of clinical states to certain disorders whose nosological homogeneity should not be taken for granted. Next, the results of our own catamnestic study on a patient population selected solely on syndromatological grounds are presented. It is shown that no prognostic significance can be attributed to the delusional pictures classified according to their description (paranoic syndromes, systematic and unsystematized paraphrenia); on the other hand, such significance certainly falls to one of the axial syndromes formulated on axis 4 (the endogenomorphic-cyclothymic axial syndrome). Nevertheless, a relatively large portion of the patients eluded attribution to one of the axial syndromes, even after a course of several years. Aside from the possibility that a number of them could be so classified after further cross-sectional evaluation, one may suspect that this group contains patients whose delusional formation stems from a psychogenic etiology; or it may be that these patients represent a third illness entity, distinct from the other two groups of endogenous psychoses.