Psychiatric day hospital treatment concepts have to deal with a wide spectrum of mental disorders. We raised the question, if day hospitals can be differentiated concerning their treatment concepts and if so how much this is reflected in their structural and procedural features. In 1999 a survey was initiated concerning structure, concept and method of treatment in psychiatric day hospitals for adults in Germany. Furthermore data concerning rate of utilization, patients' characteristics and aspects of referral and further treatment were ascertained. One hundred and seventy-three (63.4%) of 273-day hospitals contacted took part in the inquiry. The data were interpreted using multivariate as well as non-parametric procedures. The results show that treatment concepts of day hospitals can be specified as three main areas of function (psychotherapy, crisis intervention orientated treatment alternative, rehabilitation) and four therapeutic orientations (psychodynamic social psychiatric, behavioral social psychiatric, psychodynamic, sociotherapeutic). Structural features are predominantly comparable and the differences found concerning the treatment concepts are especially related to patients' characteristics and some procedural features. The conclusion is that the differentiation of day hospital treatment concepts should be taken into consideration in planning psychosocial treatment services as well as in day hospital evaluation research.