The introduction of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III), which uses a descriptive approach to operational definitions of psychiatric diagnostic groups, is an important step forward. Still the present classification systems of DSM or the International Classification of Diseases (ICD) are preliminary. A systematic descriptive approach based on data collection as a foundation for diagnostic definitions is strongly recommended. Data-based definitions have a good chance of surviving later changes of diagnostic fashion. Treatment studies will be more valuable when the course of a disorder is included in its definition. The diagnosis of depression has to include operationalized subthreshold syndromes, such as minor depression and recurrent brief depression. In the face of the elementary importance of the distinction between uni- and bipolar disorders, a development of the definition of hypomania is proposed. Based on this definition, bipolar disorders are more prevalent in the normal population than is commonly assumed.
Most diagnostic subgroups of depression are artificial. They are not diagnostic entities, but subtypes of the same spectrum disorder. During his/her lifetime, a patient may therefore receive two or more diagnoses, but still suffer from a single disorder.
Dual diagnoses, such as double depression and combined depression, define more severely affected patients with major depression, characterized by higher suicide attempt rates. Thus the differentiation of subgroups is more than an academic exercise but has a practical use.