We mailed monthly questionnaires regarding fibromyalgia symptoms to 75 patients during a one-year period. Fibromyalgia symptoms in individual patients were generally stable over time as assessed by repeated measures and slope analyses, but patients clearly differed from one another in symptom severity. Pain, psychological status, and functional disability, but not sleep disturbance or morning stiffness, were the independent explanatory factors for disease severity in regression models. Functional disability has not been recognized previously as an important factor in fibromyalgia severity, but our data suggest that it should be assessed as a process and outcome measure in future fibromyalgia studies.