In an earlier paper (Goldberg et al. 1987) 36 common symptoms of minor psychiatric disorder in general practice were analysed using the technique of latent trait analysis. From this analysis two dimensions of illness emerged, corresponding to anxiety and depression. In the present paper, this symptom-based representation of minor psychiatric illness is used as a framework for comparing four diagnostic systems: General Practitioner (GP) diagnoses, the ID-CATEGO diagnostic system, the DSM-III system and the Bedford College diagnostic system. This analysis clarifies the reasons for disagreement among systems of diagnostic criteria and examines the practical effects of alternative diagnostic algorithms.