Psychiatric symptoms are heterogeneous and differ in origin, structure and clinical expression. These differences are frequently ignored both clinically and in research. Thus, patients may be described as being anxious or as having delusions, with little realisation that different aspects of the structure of symptoms are being depicted. Neglect of differences in structure between symptoms has also naturally resulted in the neglect of differences in structure between superficially 'same' symptoms. A model is offered here which provides a means of classifying heterogeneity on the basis of five levels of clinical differentiation which, in turn, carries implications for underlying symptom structure. At the 1st level, symptoms can be differentiated in terms of the conventional category 'form' but which is in fact a composite of criteria. At the 2nd level, symptoms may be separated by differences in diagnosis which may alter structure; it is suggested that one way of capturing these is to assess the qualitative dimensions of the form. At the 3rd level, differentiation can occur on the basis of sensory modality, and, using hallucinations as an illustration, it has been shown that this is a weak and confused criterion. At the 4th level, symptoms can be differentiated on the basis of abstract criteria, often of historical origin, for which there is little empirical evidence. At the 5th level, the main criterion is difference in content. That not all symptoms will be susceptible to a fivefold analysis reinforces the argument that symptoms are structurally different and that these differences have psychometric and research implications.