OBJECTIVE: To describe the natural history and disease progression of the eosinophilia-myalgia syndrome and to assess the therapeutic effects of orally administered steroids on the disorder as of October 1990. DESIGN: Case-series analysis. A cohort of 45 patients with the eosinophilia-myalgia syndrome was followed prospectively by periodic telephone interviews and medical examinations for an average of 14 months after onset of illness. SETTING: Washington state. PATIENTS: The cases of 47 patients were reported to the Washington State Department of Health from 1 July to 12 December 1989. Two patients were unavailable for follow-up, and the remaining 45 completed the study. MAIN RESULTS: Patients were predominantly non-Hispanic white women (87%) with an average age of 49 years. Symptoms typically progressed from early onset of myalgia and fatigue to later development of neurologic and scleroderma-like skin changes. Six (13%) patients recovered completely within 2 to 5 months of symptom onset. After 14 months of illness, over half of the patients who initially presented with myalgia, fatigue, or scleroderma-like skin changes remained symptomatic. The average severity of each major symptom was measured using interviews and patient self-reports and has improved subjectively by at least 40%. Statistical analyses showed no significant difference in long-term symptom duration or severity between patients treated and those not treated with prednisone. CONCLUSIONS: The eosinophilia-myalgia syndrome is a long-term illness characterized by progressive improvement during the first 25 weeks after symptom onset, followed by a protracted phase of symptom resolution. We could not show a clear-cut benefit of prednisone in reducing the long-term severity or duration of the disease.