Background: Oral anticancer agents (OAAs) can be effective in cancer control, but patients should conduct adequate self-care in managing OAAs and related toxicities to improve their quality of life. However, tools are lacking for measuring self-care behaviours in cancer patients on OAAs. The aim of this study was to develop a new selfreported instrument, the Self-Care of Oral Anticancer Agents Index (SCOAAI) and to test its content validity.Methods: SCOAAI items were developed according to the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) criteria. The Middle Range Theory of Self-Care of Chronic Illnesses was used to guide item development, including self-care maintenance (i.e. behaviours to maintain condition stability), self-care monitoring (i.e. behaviours to monitor OAAs side effects) and selfcare management (i.e. patients' responses to deal with OAAs side effects). A literature review and qualitative interviews with patients and healthcare professionals were conducted to develop SCOAAI-specific items and to assess their comprehensibility and comprehensiveness. The Content Validity Index (CVI) was calculated.Results: A first version of the SCOAAI included 44 items. Patients participating in the cognitive interviews (n¼12; n¼6 women, mean age 65 years [SD AE 8]; 37.5% with lung cancer) deemed the instructions, items and response options to be comprehensible and comprehensive. Face-to-face interviews with 5 experts (n¼3 female, mean age: 52 years [SD AE 12.28], n¼4 physicians) addressed some issues of the instrument, resulting in removal of 12 items. The final version of the SCOAAI included 32 items: 15 in self-care maintenance, 11 in self-care monitoring and 6 in self-care management. The item CVI ranged between 0.8 and 1; the scale CVI was 0.99. Conclusions:The SCOAAI showed good content validity and is a promising and psychometrically sound instrument for the assessment of self-care behaviours of cancer patients on OAAs. Since better self-care is associated with better clinical outcomes (e.g. less toxicity, better quality of life and fewer hospitalisations), the SCOAAI, once validated, could become a useful instrument for use in oncology clinical practice and research.Legal entity responsible for the study: M. Di Nitto.Funding: "Lega Italiana per la Lotta contro i Tumori" (LILT) foundation.