Purpose <p>The PErceived CAusal Network (PECAN) approach offers a novel, idiographic method for mapping perceived causal relations between symptoms. While promising for clinical case formulation, prior studies have shown limited test-retest reliability and have been based on self-reported or non-clinical samples. This observational two-wave study examined the feasibility, test-retest reliability, and subjective clinical utility of a structured, interview-based PECAN assessment in patients with Major Depressive Disorder (MDD) and Fibromyalgia Syndrome (FMS).</p> Methods <p>A structured symptom list was developed through literature review and expert and patient input. A total of 100 clinically diagnosed adult patients (<i>n</i> = 50 MDD; <i>n</i> = 50 FMS) completed two semi-structured PECAN interviews, spaced one week apart. Participants selected ten personally relevant symptoms from the pre-defined list of symptoms, and rated the perceived causal influence between these. Feasibility and clinical utility were assessed via standardized feedback; test-retest reliability was examined using Spearman correlations for edge weights and centrality indices. Qualitative data were analyzed to identify reasons for inconsistencies between interviews.</p> Results <p>The PECAN interviews were rated as comprehensible and manageable. Mean test-retest reliability was <i>r</i> = .62 for edge weights and <i>r</i> = .76 for out-degree centrality. Qualitative inconsistencies were mainly attributed to symptom variation (50%) or methodological factors (26%). Most participants reported increased awareness of symptom interactions.</p> Conclusions <p>The PECAN interview proved to be a feasible and acceptable method for assessing perceived symptom relationships in clinically diagnosed populations. Further refinement is needed to improve measurement precision and support robust idiographic case conceptualizations.</p>