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AIMS The aim of the study was to develop and psychometrically test the Nurse-Patient Mutuality in Chronic Illness (NPM-CI) scale in two versions, one for nurses and one for patients. METHOD A multiphase methodological study was conducted. In the first phase, a qualitative investigation was conducted through interviews and an analysis of the content; inductively, the items of two instruments were generated, one for nurses and one for patients. In the second phase, the content and face validity were assessed through the expert consensus method. In the third phase, exploratory factor analysis (EFA), Cronbach's alpha test, intraclass correlation and Pearson correlation coefficients were conducted to estimate construct, criterion validity and instrument reliability. For each phase, the sample included nurses and patients recruited from a large hospital in Northern Italy. Data collection was conducted between June and September 2021. RESULTS Nurse and patient versions of the NPM-CI scale were developed. Two rounds of consensus reduced the items from 39 to 20; content validity index ranged between 0.78 and 1, content validity ratio was 0.94. Face validity indicated clarity and comprehensibility of the items. EFA identified three latent factors for both the scales. Internal consistency was satisfactory, with Cronbach's alphas ranging between .80 and .90. Test-retest stability was suggested, with an intraclass correlation coefficient of .96 (nurse scale) and .97 (patient scale). Predictive validity was established, with a Pearson correlation coefficient of .43 (nurse scale) and 0.55 (patient scale) between the mutuality scales and satisfaction in providing and receiving care. CONCLUSION The results suggest that the NPM-CI scales are sufficiently valid and reliable for the clinical practice among chronic illness patients and the nurses caring for them. A more in-depth exploration of this construct in the context of nursing and patient outcomes is warranted. PATIENT OR PUBLIC CONTRIBUTION Patients were involved in all study phases. IMPACT Mutuality is fundamental in the relationship between nurse and patient, based on trust, equality, reciprocity, and mutual respect. The NPM-CI scale was developed and psychometric estimated through a multiphase study in both nurse and patient versions. The NPM-CI scale measures the factors of 'developing and going beyond', 'being the point of reference' and 'deciding and sharing care'. The NPM-CI scale allows us to measure mutuality in clinical practice and research. Expected outcomes and influencing factors for patients and nurses could be associated.