We examined between and within-person variability, affective correlates, and diagnostic differences in daily fatigue in women with rheumatoid arthritis (RA), osteoarthritis (OA), and fibromyalgia syndrome (FMS). Two hundred and fifty-five female patients recruited from the community served as participants for this project. The patients had a physician-confirmed diagnosis of RA (n = 89), OA (n = 76), or FMS (n = 90). Individuals completed an initial questionnaire and up to 32 daily diaries assessing illness symptoms and psychosocial variables (i.e., fatigue, pain, sleep problems, depression, and affect). The primary outcome for the current project was variability in fatigue. We examined affective, pain, and sleep correlates of fatigue, and tested whether these relations varied by diagnosis. Results indicated that FMS patients had higher overall levels of and greater daily variability in fatigue compared with the other pain groups. For all patients, fatigue correlated highly with lower positive affect (PA). Moreover, day-to-day increases in fatigue were associated with decreases in PA, particularly among FMS patients, and with increases in negative affect (NA). Daily pain was associated with increased fatigue in all groups, although OA patients showed less pain reactivity than either FMS or RA patients. These findings indicate that fatigue is a common feature of rheumatologic conditions. Nonetheless, there are important differences between RA, OA, and FM patients in both the everyday manifestations and the biopsychosocial correlates of fatigue.