Fatigue is a well-recognized complaint with major impact on daily life in primary Sjögren's syndrome (PSS) and systemic lupus erythematosus (SLE). Previous research has not taken into account several crucial aspects of fatigue. This study examined various aspects of fatigue in the daily life of patients with PSS and SLE and in healthy controls. We compared age-adjusted, repeated measurements of fatigue across the day of female patients with SLE (n = 20, mean age 43.4 +/- 11.3), with PSS (n = 28, mean age 53.7 +/- 13.9) and healthy participants (n = 30, mean age 50.5 +/- 13.4). General and physical fatigue was significantly higher in patients than in healthy participants. Groups did not differ with respect to average levels of reduced motivation or mental fatigue. Both general and physical fatigue and reduced activity varied significantly during the day. Adjusting for depressive symptoms, groups showed significantly different time courses during the day. In healthy participants and patients with SLE, fatigue first decreased and then increased, whereas a rather opposite course-at least for the first part of the day-was observed in patients with PSS. Using an ecologically valid assessment method, we demonstrated substantially higher levels of daily fatigue in SLE and PSS patients as compared to healthy participants, thereby jeopardizing these patients' quality of life. The effect of disease on variations in fatigue over the day should be the subject of further inquiry, especially as it might clarify underlying mechanisms.