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Background: Patient uncertainty is considered to be an inherent part of the illness experience, and particularly relevant in unpredictable conditions; however, it has not been thoroughly investigated in systemic lupus erythematosus (SLE) or rheumatoid arthritis (RA) and no appropriate instrument is available for its quantification. This thesis presents mixed-method studies aiming to address this gap in the literature. Phase-1: Qualitative interviews with 32 patients and 8 health care professionals were conducted in order to conceptualise patient uncertainty in SLE and RA. These findings were used to develop a new self-report instrument for patient uncertainty. Items of the new instrument were qualitatively tested through cognitive debriefing interviews. Phase-2: A field test was set up to evaluate and revise the newly developed instrument psychometrically, using the modern technique of Rasch analysis in a sample of 388 patients. The instrument was subsequently evaluated using traditional psychometrics tests. Phase-3 (part-1): A second field test was set up to evaluate the psychometric properties of the second draft of the new instrument using a combination of modern and traditional psychometric techniques in an independent sample of 279 patients. The final draft of the instrument consisted of five scales; symptoms and flares, medication, trust in doctor, self-management and impact. Phase-3 (part-2): The construct validity of the new instrument, as well as the contribution of the five patient uncertainty scales to SLE and RA patient outcomes, including treatment adherence, mood and health related quality of life, were explored. Statistical tests, including correlational analyses and multiple linear regressions, were used for this exploration. Conclusions: This thesis offers a conceptual framework and a self-report instrument for the assessment of patient uncertainty in SLE and RA. The findings offer implications for the role of patient uncertainty in these conditions and demonstrate the importance of comprehensive methodology in assessing such constructs.