Objective: The INTERMED Self-Assessment questionnaire (IMSA) was developed as an alternative to the observer-rated INTERMED (IM) to assess biopsychosocial complexity and health care needs. We studied feasibility, reliability, and validity of the IMSAwithin a large and heterogeneous international sample of adult hospital inpatients and outpatients as well as its predictive value for health care use (HCU) and quality of life (QoL). Methods: A total of 850 participants aged 17 to 90 years from five countries completed the IMSA and were evaluated with the IM. The following measurement properties were determined: feasibility by percentages of missing values; reliability by Cronbach a; interrater agreement by intraclass correlation coefficients; convergent validity of IMSA scores with mental health (Short Form 36 emotional well-being subscale and Hospital Anxiety and Depression Scale), medical health (Cumulative Illness Rating Scale) and QoL (Euroqol-5D) by Spearman rank correlations; and predictive validity of IMSA scores with HCU and QoL by (generalized) linear mixed models. Results: Feasibility, face validity, and reliability (Cronbach a = 0.80) were satisfactory. Intraclass correlation coefficient between IMSA and IM total scores was.78 (95% CI =.75-.81). Correlations of the IMSAwith the Short Form 36, Hospital Anxiety and Depression Scale, Cumulative Illness Rating Scale, and Euroqol-5D (convergent validity) were-.65,.15,.28, and-.59, respectively. The IMSA significantly predicted QoL and also HCU (emergency department visits, hospitalization, outpatient visits, and diagnostic examinations) after 3-and 6-month follow-up. Results were comparable between hospital sites, inpatients and outpatients, as well as age groups. Conclusions: The IMSA is a generic and time-efficient method to assess biopsychosocial complexity and to provide guidance for multidisciplinary care trajectories in adult patients, with good reliability and validity across different cultures.