The importance of family and friends for patients with mental health problems has been recognized over the past few years. Significant others (SO) of patients admitted to a psychiatric ward specializing in affective disorders and suicide prevention filled in a self-rating questionnaire concerning their burden as SOs, the Involvement Evaluation Questionnaire (IEQ). The aims of this study were twofold: first, to test whether the questionnaire IEQ was applicable in psychiatric services in Sweden and second, to relate the IEQ results to socio-demographic data and diagnosis of the patients, and also to the situation of the SOs. We found the questionnaire useful. Nearly half of the SOs had difficulties in accepting and handling the patient's psychiatric problems. More than half had been worried that the patient was going to harm him/herself and because of this had kept close watch on the patient. Almost all SOs worried about the patient's general health and future. Most of them felt obliged to intervene in the patient's activities of daily living. More than half worried about the kind of medical and psychological treatment the patient was receiving. Most SOs concluded that they were burdened by their engagement in the patient. Our results were compared with a European study of schizophrenic patients also investigated with the IEQ. The comparison indicated that the perceived burden on SOs of these different kinds of psychiatric patients can be equally burdensome. SOs need information, help and support in their difficult support role. This questionnaire could be useful in finding new routines for increasing the involvement by SOs in the treatment of patients with mental health problems.