An unusual, nonfatal illness characterized by a prolonged, relapsing course occurred over a seven-month period in 26 of 69 nuns (38%). Symptoms were fatigue, weakness, low-grade fever, nausea, headache and pain in trunk or limbs, emotional instability, depression, impaired thinking, and paresthesias. Abnormal laboratory findings in a few cases included elevated spinal-fluid cell counts, relative lymphocytosis in the peripheral blood, and elevated erythrocyte sedimentation rate. Marked creatinuria was found in four patients studied for this abnormality. Abnormal creatine excretion suggests that muscle involvement is an important and objective finding.