OBJECTIVE: Little is known about the relationship between morning symptoms of rheumatoid arthritis (RA) and measures of disease activity currently used to assess RA. Information available from the Circadian Administration of Prednisone in Rheumatoid Arthritis (CAPRA-2) study was used to investigate these relationships. METHODS: CAPRA-2 included 350 patients with RA who were symptomatic despite treatment with disease-modifying antirheumatic drugs, randomized 2:1 to additional treatment with a 5-mg daily dose of delayed-release prednisone or placebo. Pearson's correlations were used to evaluate the relationships between change from baseline in symptoms (duration of morning stiffness, severity of morning stiffness, and intensity of pain on waking) and measures of disease activity (the American College of Rheumatology 20% improvement criteria [ACR20], the Disease Activity Score in 28 joints [DAS28], and the Health Assessment Questionnaire disability index). Correlations were defined as weak (<0.3), moderate (0.3-0.7), or strong (>0.7). RESULTS: There was a strong correlation between the severity of morning stiffness and the intensity of morning pain (Pearson's correlation 0.91, P < 0.001). There was a weak correlation between the duration of morning stiffness and measures of disease activity (0.24-0.28), with moderate correlations between the severity of morning stiffness or intensity of pain on waking and DAS28 or ACR20 scores (0.44-0.48). Severity of morning stiffness showed less variability and a greater effect size than did duration of morning stiffness. CONCLUSION: Morning symptoms and measures of disease activity show weak to moderate correlations. Severity of morning stiffness showed less variability and greater effect size than did duration of morning stiffness. These findings suggest that severity is the preferred construct to measure the impact of morning stiffness in patients with RA, information that is not fully captured in the RA core set.